Thanks for your nice explanation, ikegami.

By the way, I think you want
"s{\\Q\$in\\E}{\$out}g"
instead of
"s{\\Q$in\\E}{$out}g"
The difference will be seen when $in or $out contains a '$'.
[download]

Yes, it is evident from your explanation that that would be the case. I did not post the rest of the code, but my $in and $out are a finite set of strings, neither containing a '$'.