use File::Spec;

my ($volume, $dirstring, $file) = File::Spec->splitpath($pathname);
my @dirs = File::Spec->splitdir($dirstring);

    # Unfortunately, splitpath's doc says:
    # The directory portion may or may not be returned with a trailing '/'.
    #
    # Which 'may or may not' give us an extra 'null' member of
    # @dirs, because splitdir allows null directory names. 
    #
    # Hence this next line, purely for cleanup:

pop @dirs if ($dirs[-1] eq '');

$file =~ s/\.pm$//;
my $package = join('::',@dirs,$file);