I don't know why you should be experiencing the regex oddity, but it does seem to me that this is likely to be extremely slow on long strings, because of the huge amount of backtracking that could be required. Would

@count{ qw/o r w/ } = ( tr/oO/oO/, tr/rR/rR/, tr/wW/wW/ );
[download]

(preceded by my %count, of course, and followed by appropriate testing) do what you need?