Not to sound facetious, but several other posters and I basically answered this question(which is almost identical to your original post) by saying: "Don't do that!" I know it's Perl and TIMTOWTDI, but sometimes the way you are doing it is wrong. Yet again the correct way to write the sub above is:

sub chg {
   my ($str, $pat) = @_;
   eval "\$str =~ tr/a$1/$1a/" if $str =~ /$pat/;
   return $str;
}
[download]

As far as debugging other people's code which is coded like this. I suppose firing up the perl debugger and stepping through the program may shed some light, although something as obscure as your example which only fails with some singularly weird input may be very difficult to figure out. However, if you just shuffle around your house hitting yourself in the head with your Camel book and repeating the mantra, "Never check $1 blindly, make certain my regex succeeded." You will, after much meditation and contemplation, literally "beat" it into your head so whenever you see $1 in someone's code, you will instantly make sure it is properly initialized before it is used.

HTH