Perl in a Nutshell, 2nd Edition

Subroutines

Subroutines are declared using one of these forms:

sub name {block}
sub name (proto) {block}

Prototypes allow you to put constraints on the arguments you provide to your subroutines.

You can also create anonymous subroutines at runtime, which will be available for use through a reference:

$subref = sub {block};