Subroutines need a declaration, i.e., a specification of how they should be called, and a definition, i.e., a specification of what they should do when called.
- sub name [
(proto)] [ attributes ]; Declares name as a subroutine, optionally specifying the prototype and attributes. Declaring a subroutine is optional, but allows the subroutine to be called just like Perl’s built-in operators.
- sub [ name ] [
(proto)] [ attributes ] block Defines subroutine name, with optional prototype and attributes. If the subroutine has been declared with a prototype or attributes, the definition should have the same prototype and attributes. When name is omitted, the subroutine is anonymous and the definition returns a reference to the code.
When a subroutine is called, the statements in block are executed. Parameters are passed as a flat list of scalars as array @_. The elements of @_ are aliases for the scalar parameters. The call returns the value of the last expression evaluated. wantarray (page 11) can be used to determine the context in which the subroutine was called.
Subroutines that have an empty prototype and do nothing but return a fixed value are inlined, e.g., sub PI() { 3.1415 }. See also the subsection Prototypes.
attributes are introduced with a : (colon). Perl supports the following attributes:
| The subroutine returns a scalar variable that can be assigned to. |
| The subroutine is a method. |
There are several ways to call a subroutine.
- name
([ parameters ]) The most common way. The parameters are passed by reference as array
@_.- &name
([ parameters ]) Prototype specifications, if any, are ignored.
- &name
The current
@_is passed directly to the subroutine.- name [ arguments ]
If the subroutine has been declared, or defined, it may be called as a built-in operator, without parentheses.
- caller [ expr ]
Returns a list (package, file, line) for a specific subroutine call.
callerreturns this information for the current subroutine, With expr, returns an extended list;caller(0)for this subroutine,caller(1)for the subroutine that called this subroutine, etc. See the table on the following page.Returns false if no caller.
Elements of the extended result list:
Index
Name
Description
0
package
The calling package.
1
filename
The filename.
2
line
The line number.
3
subroutine
The name of the subroutine.
4
hasargs
True if arguments were supplied.
5
wantarray
Call context.
6
evaltext
Text if called by eval.
7
is_require
Called by use or require.
8
hints
Pragmatic hints.
9
bitmask
Pragmatic hints.
10
hinthash
Pragmatic hints.
- defined &name
Tests whether the named subroutine has been defined (has a body).
- do name list
Deprecated form of
&name.- exists &name
True if the named subroutine has been declared, either with a body or with a forward declaration.
- goto
&name Substitutes a call to name for the current subroutine.
- return [ expr ]
Returns from a subroutine, eval, or do file with the value specified. Without expr, returns undef in scalar context and an empty list in list context.
Meaning | |
|---|---|
| Enforces scalar context. |
| Enforce list context; all remaining arguments are consumed. |
| Accepts a bare name, a constant, a scalar expression, a typeglob, or a reference to a typeglob. |
| Requires a code (subroutine) reference. |
| The argument must start with a |
| The argument must start with a |
| The argument must start with a |
| The argument must start with any of the specified sigils. |
(Empty parentheses) If the subroutine returns a fixed value the call is inlined. | |
| Requires a scalar expression, defaults to |
| Requires an array, a hash, or a reference. |
| Indicates that subsequent arguments are optional. |
- prototype name
Returns the prototype for the named function as a string, or undef if the function has none.
Use
CORE::name for built-in functions.