Good catch on the empty alternative! As far as I can tell it's there from the beginning when I tried to write the three tests in one regex. I missed it when I split them up to make them clearer. It doesn't hurt anything, however.

The line push @queue, "$`#$'" =~ s/$&/\l$&/gr while /$&/g; made me giggle a little when I wrote it :)
I often giggle when writing perl, it's one heck of a great language.