perlsyn - Perl syntax

A Perl program consists of a sequence of declarations and statements which run from the top to the bottom. Loops, subroutines and other control structures allow you to jump around within the code.

Perl is a free-form language, you can format and indent it however you like. Whitespace mostly serves to separate tokens, unlike languages like Python where it is an important part of the syntax.

Many of Perl's syntactic elements are optional. Rather than requiring you to put parentheses around every function call and declare every variable, you can often leave such explicit elements off and Perl will figure out what you meant. This is known as Do What I Mean, abbreviated DWIM. It allows programmers to be lazy and to code in a style with which they are comfortable.

Perl borrows syntax and concepts from many languages: awk, sed, C, Bourne Shell, Smalltalk, Lisp and even English. Other languages have borrowed syntax from Perl, particularly its regular expression extensions. So if you have programmed in another language you will see familiar pieces in Perl. They often work the same, but see perltrap for information about how they differ.

The only things you need to declare in Perl are report formats and subroutines (and sometimes not even subroutines). A variable holds the undefined value (undef) until it has been assigned a defined value, which is anything other than undef. When used as a number, undef is treated as 0; when used as a string, it is treated as the empty string, ""; and when used as a reference that isn't being assigned to, it is treated as an error. If you enable warnings, you'll be notified of an uninitialized value whenever you treat undef as a string or a number. Well, usually. Boolean contexts, such as:

are exempt from warnings (because they care about truth rather than definedness). Operators such as ++, --, +=, -=, and .=, that operate on undefined left values such as:

are also always exempt from such warnings.

A declaration can be put anywhere a statement can, but has no effect on the execution of the primary sequence of statements--declarations all take effect at compile time. Typically all the declarations are put at the beginning or the end of the script. However, if you're using lexically-scoped private variables created with my(), you'll have to make sure your format or subroutine definition is within the same block scope as the my if you expect to be able to access those private variables.

Declaring a subroutine allows a subroutine name to be used as if it were a list operator from that point forward in the program. You can declare a subroutine without defining it by saying sub name, thus:

Note that myname() functions as a list operator, not as a unary operator; so be careful to use or instead of || in this case. However, if you were to declare the subroutine as sub myname ($), then myname would function as a unary operator, so either or or || would work.

Subroutines declarations can also be loaded up with the require statement or both loaded and imported into your namespace with a use statement. See perlmod for details on this.

A statement sequence may contain declarations of lexically-scoped variables, but apart from declaring a variable name, the declaration acts like an ordinary statement, and is elaborated within the sequence of statements as if it were an ordinary statement. That means it actually has both compile-time and run-time effects.

Text from a "#" character until the end of the line is a comment, and is ignored. Exceptions include "#" inside a string or regular expression.

The only kind of simple statement is an expression evaluated for its side effects. Every simple statement must be terminated with a semicolon, unless it is the final statement in a block, in which case the semicolon is optional. (A semicolon is still encouraged if the block takes up more than one line, because you may eventually add another line.) Note that there are some operators like eval {} and do {} that look like compound statements, but aren't (they're just TERMs in an expression), and thus need an explicit termination if used as the last item in a statement.

The number 0, the strings '0' and '', the empty list (), and undef are all false in a boolean context. All other values are true. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as '', but as a number, it is treated as 0.

Any simple statement may optionally be followed by a SINGLE modifier, just before the terminating semicolon (or block ending). The possible modifiers are:

The EXPR following the modifier is referred to as the "condition". Its truth or falsehood determines how the modifier will behave.

if executes the statement once if and only if the condition is true. unless is the opposite, it executes the statement unless the condition is true (i.e., if the condition is false).

The foreach modifier is an iterator: it executes the statement once for each item in the LIST (with $_ aliased to each item in turn).

while repeats the statement while the condition is true. until does the opposite, it repeats the statement until the condition is true (or while the condition is false):

The while and until modifiers have the usual "while loop" semantics (conditional evaluated first), except when applied to a do-BLOCK (or to the deprecated do-SUBROUTINE statement), in which case the block executes once before the conditional is evaluated. This is so that you can write loops like:

See "do" in perlfunc. Note also that the loop control statements described later will NOT work in this construct, because modifiers don't take loop labels. Sorry. You can always put another block inside of it (for next) or around it (for last) to do that sort of thing. For next, just double the braces:

For last, you have to be more elaborate:

NOTE: The behaviour of a my statement modified with a statement modifier conditional or loop construct (e.g. my $x if ...) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons.

In Perl, a sequence of statements that defines a scope is called a block. Sometimes a block is delimited by the file containing it (in the case of a required file, or the program as a whole), and sometimes a block is delimited by the extent of a string (in the case of an eval).

But generally, a block is delimited by curly brackets, also known as braces. We will call this syntactic construct a BLOCK.

The following compound statements may be used to control flow:

Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets are required--no dangling statements allowed. If you want to write conditionals without curly brackets there are several other ways to do it. The following all do the same thing:

The if statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is never any ambiguity about which if an else goes with. If you use unless in place of if, the sense of the test is reversed.

The while statement executes the block as long as the expression is true (does not evaluate to the null string "" or 0 or "0"). The until statement executes the block as long as the expression is false. The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements next, last, and redo. If the LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the LABEL. Such desperate behavior triggers a warning if you use the use warnings pragma or the -w flag.

If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated again. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement.

The next command starts the next iteration of the loop:

The last command immediately exits the loop in question. The continue block, if any, is not executed:

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed. This command is normally used by programs that want to lie to themselves about what was just input.

For example, when processing a file like /etc/termcap. If your input lines might end in backslashes to indicate continuation, you want to skip ahead and get the next record.

which is Perl short-hand for the more explicitly written version:

Note that if there were a continue block on the above code, it would get executed only on lines discarded by the regex (since redo skips the continue block). A continue block is often used to reset line counters or ?pat? one-time matches:

If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested before the first iteration.

The loop control statements don't work in an if or unless, since they aren't loops. You can double the braces to make them such, though.

This is caused by the fact that a block by itself acts as a loop that executes once, see "Basic BLOCKs and Switch Statements".

The form while/if BLOCK BLOCK, available in Perl 4, is no longer available. Replace any occurrence of if BLOCK by if (do BLOCK).

Perl's C-style for loop works like the corresponding while loop; that means that this:

is the same as this:

There is one minor difference: if variables are declared with my in the initialization section of the for, the lexical scope of those variables is exactly the for loop (the body of the loop and the control sections).

Besides the normal array index looping, for can lend itself to many other interesting applications. Here's one that avoids the problem you get into if you explicitly test for end-of-file on an interactive file descriptor causing your program to appear to hang.

Using readline (or the operator form, <EXPR>) as the conditional of a for loop is shorthand for the following. This behaviour is the same as a while loop conditional.

The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn. If the variable is preceded with the keyword my, then it is lexically scoped, and is therefore visible only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value upon exiting the loop. If the variable was previously declared with my, it uses that variable instead of the global one, but it's still localized to the loop. This implicit localisation occurs only in a foreach loop.

The foreach keyword is actually a synonym for the for keyword, so you can use foreach for readability or for for brevity. (Or because the Bourne shell is more familiar to you than csh, so writing for comes more naturally.) If VAR is omitted, $_ is set to each value.

If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over.

If any part of LIST is an array, foreach will get very confused if you add or remove elements within the loop body, for example with splice. So don't do that.

foreach probably won't do what you expect if VAR is a tied or other special variable. Don't do that either.

Examples:

Here's how a C programmer might code up a particular algorithm in Perl:

Whereas here's how a Perl programmer more comfortable with the idiom might do it:

See how much easier this is? It's cleaner, safer, and faster. It's cleaner because it's less noisy. It's safer because if code gets added between the inner and outer loops later on, the new code won't be accidentally executed. The next explicitly iterates the other loop rather than merely terminating the inner one. And it's faster because Perl executes a foreach statement more rapidly than it would the equivalent for loop.

A BLOCK by itself (labeled or not) is semantically equivalent to a loop that executes once. Thus you can use any of the loop control statements in it to leave or restart the block. (Note that this is NOT true in eval{}, sub{}, or contrary to popular belief do{} blocks, which do NOT count as loops.) The continue block is optional.

The BLOCK construct is particularly nice for doing case structures.

There is no official switch statement in Perl, because there are already several ways to write the equivalent.

However, starting from Perl 5.8 to get switch and case one can use the Switch extension and say:

after which one has switch and case. It is not as fast as it could be because it's not really part of the language (it's done using source filters) but it is available, and it's very flexible.

In addition to the above BLOCK construct, you could write

(That's actually not as strange as it looks once you realize that you can use loop control "operators" within an expression. That's just the binary comma operator in scalar context. See "Comma Operator" in perlop.)

or

or formatted so it stands out more as a "proper" switch statement:

or

or even, horrors,

A common idiom for a switch statement is to use foreach's aliasing to make a temporary assignment to $_ for convenient matching:

Another interesting approach to a switch statement is arrange for a do block to return the proper value:

Or

Or if you are certain that all the && clauses are true, you can use something like this, which "switches" on the value of the HTTP_USER_AGENT environment variable.

That kind of switch statement only works when you know the && clauses will be true. If you don't, the previous ?: example should be used.

You might also consider writing a hash of subroutine references instead of synthesizing a switch statement.

Although not for the faint of heart, Perl does support a goto statement. There are three forms: goto-LABEL, goto-EXPR, and goto-&NAME. A loop's LABEL is not actually a valid target for a goto; it's just the name of the loop.

The goto-LABEL form finds the statement labeled with LABEL and resumes execution there. It may not be used to go into any construct that requires initialization, such as a subroutine or a foreach loop. It also can't be used to go into a construct that is optimized away. It can be used to go almost anywhere else within the dynamic scope, including out of subroutines, but it's usually better to use some other construct such as last or die. The author of Perl has never felt the need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:

The goto-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. This is used by AUTOLOAD() subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to @_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller() will be able to tell that this routine was called first.

In almost all cases like this, it's usually a far, far better idea to use the structured control flow mechanisms of next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of eval{} and die() for exception processing can also be a prudent approach.

Perl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a new statement, if the compiler encounters a line that begins with an equal sign and a word, like this

Then that text and all remaining text up through and including a line beginning with =cut will be ignored. The format of the intervening text is described in perlpod.

This allows you to intermix your source code and your documentation text freely, as in

Note that pod translators should look at only paragraphs beginning with a pod directive (it makes parsing easier), whereas the compiler actually knows to look for pod escapes even in the middle of a paragraph. This means that the following secret stuff will be ignored by both the compiler and the translators.

You probably shouldn't rely upon the warn() being podded out forever. Not all pod translators are well-behaved in this regard, and perhaps the compiler will become pickier.

One may also use pod directives to quickly comment out a section of code.

Perl can process line directives, much like the C preprocessor. Using this, one can control Perl's idea of filenames and line numbers in error or warning messages (especially for strings that are processed with eval()). The syntax for this mechanism is the same as for most C preprocessors: it matches the regular expression

with $1 being the line number for the next line, and $3 being the optional filename (specified with or without quotes).

There is a fairly obvious gotcha included with the line directive: Debuggers and profilers will only show the last source line to appear at a particular line number in a given file. Care should be taken not to cause line number collisions in code you'd like to debug later.

Here are some examples that you should be able to type into your command shell: