Programming Perl, 4th Edition

Syntax

To declare a named subroutine without defining it, use one of these forms:

sub NAME
sub NAME PROTO
sub NAME       ATTRS
sub NAME PROTO ATTRS

To declare and define a named subroutine, add a BLOCK:

sub NAME             BLOCK
sub NAME PROTO       BLOCK
sub NAME       ATTRS BLOCK
sub NAME PROTO ATTRS BLOCK

To create an anonymous subroutine or closure, leave out the NAME:

sub                  BLOCK
sub      PROTO       BLOCK
sub            ATTRS BLOCK
sub      PROTO ATTRS BLOCK

PROTO and ATTRS stand for the prototype and attributes, each of which is discussed in its own section later in this chapter. They’re not so important—the NAME and the BLOCK are the essential parts, even when they’re missing.

For the forms without a NAME, you still have to provide some way of calling the subroutine. So be sure to save the return value since this form of sub declaration is not only compiled at compile time as you would expect, but also produces a runtime return value:

$subref = sub BLOCK;

To import subroutines defined in another module, say:

use MODULE qw(NAME1 NAME2 NAME3 ...);

To call subroutines directly, say:

NAME(LIST)            # & is optional with parentheses.
NAME LIST             # Parens optional if sub predeclared/imported.
&NAME                 # Exposes current @_ to that subroutine,
                      # (and circumvents prototypes).

To call subroutines indirectly (by name or by reference), use any of these:

&$subref(LIST)        # The & is not optional on indirect call
$subref–>(LIST)       # (unless using infix notation).
&$subref              # Exposes current @_ to that subroutine.

The official name of a subroutine includes the & prefix. A ...