My baby, ack, broke under Perl 5.10.0, because of a fix in regex behavior that I had been using unknowingly. See, I had always used my regex objects like this: 

my $re = qr/^blah blah/;
if ( $string =~ /$re/sm )...

when I should have been using it like this: 

my $re = qr/^blah blah/sm;
if ( $string =~ /$re/ )...

The bug in 5.8.x is that the /$re/sm would incorrectly apply the /sm modifiers to $re. This made the code happen to work, but for the wrong reason. What was especially tricky about finding my bug was that in 5.10.0, the call to /$re/sm ignores the /sm, but doesn't tell you that.

After some back and forth on p5p, a patch was submitted that gave the warning about the ignored /sm flags, but alas, Perl 5.10 was already out. It wouldn't have been so bad if it hadn't been the day AFTER it was released.

So, lesson learned: Test your code against new release candidates of Perl, both for your code's sake, AND for Perl's sake.

And y'know, now that I think of it, this is probably a great policy for Perl::Critic just waiting to happen. I wonder how many other people are doing their regexes the wrong way, too.