However...

If you really want to check to see if the sub is being invoked as a method or a function, you can use the following:

#!/usr/bin/perl

package myClass;
use strict;
use warnings;

### Pure-cheese useless constructor
sub new { bless {}, __PACKAGE__ }

### Bimodal sub
sub someFunction {
    my $self = shift
        if @_ && UNIVERSAL::isa( $_[0], 'UNIVERSAL' );
    my $otherArg = shift;

    print "Called as ", defined $self ? "method" : "function",
        " with '$otherArg' argument.\n";
}

package main;
use strict;
use warnings;

# Invoked as a function
myClass::someFunction( "function thingie" );

# Invoked as a method
my $testThingie = new myClass;
$testThingie->someFunction( "method thingie" );

# It even works invoked as a static method
myClass->someFunction( "static method thingie" );
[download]

It basically just uses the mother of all superclasses ('UNIVERSAL') to check for object/class-ness. If a thing can be said to inherit from UNIVERSAL (have UNIVERSAL in its @ISA), then it can be assumed to be either a blessed reference (aka object), or the name of a class. What this function doesn't test is to see if the package that the first arg belongs to is actually your own. This is left as an excercise for the reader. =:)

Like Randal said, though: Once you venture down that path, your interface forever must support such nastiness, which can only lead to grief and ruin.