Consider:

use strict;
use warnings;

my $obj = bless {};

# As function
do_something ();

# As method
$obj->do_something ();


sub do_something {
    my ($self) = @_;

    if ($self && 'main' eq ref $self) {
        print "Called as method\n";
    } else {
        print "Called as function\n";
    }
}
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Prints:

Called as function
Called as method
[download]