sub function_name {
    # for a function call
    my $self = __PACKAGE__;
    # for a method call
    $self = shift if UNIVERSAL::isa( $_[0], __PACKAGE__ );
    ...
}

##</code><code>##

sub function_name {
    my $self = UNIVERSAL::isa( $_[0], __PACKAGE__ )
        ? shift : __PACKAGE__;
    ...
}