Btw, I just noticed that if you happen to pass neither decoration nor directory, $prefix will be undefined and you'll get warnings. Also, the sub is testing definedness of $dir, but the main program is passing an empty string in the last call — doesn't match up. All that considered, the sub should probably read

sub print_diff {
    my ( $decor, $dir, @filename ) = @_;
    my $prefix = '';
    $prefix .= "$decor " if $opt_plain;
    $prefix .= "$dir " if defined $dir and length $dir; # awkward :-(
    print map "$prefix$_\n", @filename;
}
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That's what I get for not testing my code. :-)