I disagree that it's "almost always a bad idea". You just have to be careful how you do it.

{
package Foo;
use Scalar::Util 'blessed';

sub _process_calling_convention
{
    # discard pkg name, if called as class method:
    @_ && !ref($_[0]) && $_[0] eq __PACKAGE__ and shift;
    # return object, if called as object method:
    @_ && (blessed($_[0])||'') eq __PACKAGE__ ? shift : ()
}

sub bar
{
    my $obj = &_process_calling_convention;

    if ( defined $obj )
    {
        print "called as object method $obj->( @_ )\n";
    }
    else
    {
        print "called as function ( @_ )\n";
    }
}

}

# main: test:

my $x = bless {}, 'Foo';

print "\nplain module function call:\n";
Foo::bar();
Foo::bar("arg");

print "\nclass method call:\n";
Foo->bar();
Foo->bar("arg");

print "\nobject method call:\n";
$x->bar();
$x->bar("arg");
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In this, I've coded based on the presumption that, in the class method call case, you don't care about the class name; it is discarded.

I should also say that I don't know how well this will hold up under inheritance. You'd probably have to tweak certain things, i.e. rather than string test against __PACKAGE__, test using isa or can.