our %df_keys;
sub directories_first ($$) {
    $df_keys{$_[0]} ||= df_key($_[0]);
    $df_keys{$_[1]} ||= df_key($_[1]);
    return ( $df_keys{$_[0]} cmp $df_keys{$_[1]} );
}

# somewhere else in the galaxy:

my @new_array = sort Some::Package::directories_first @old_array;