Thinking some more about it; if there can more of the same subcategorie (as suggested by the <subcat>/number), then you can replace the code

        if ($name =~ s/^$subcat//) {
            $output .= " $subcat/1";
        }
[download]

with

        if ($name =~ /^$subcat/) {
            $output .= " $subcat/";
            my $c = 0;
            $c++ while $name =~ s/^$subcat//;
            $output .= $c;
        }
        # Special case: we don't want "null/null" after  
        # adding valid subcats.
        last if not $name;
[download]

This way, filenames like "clocksvertvertred001.jpg" are displayed like

clocksvertvertred001.jpg -> category: clocks; subcats: vert/2 red/1 IM#: 001

Arjen