perlre says:

NOTE: Failed matches in Perl do not reset the match variables, which makes it easier to write code that tests for a series of more specific cases and remembers the best match.

However, we both remember something different about that.
I hope some other Monk can point us in the right direction.

Update: After some reflection, my memory recalls some situation where there were several capture groups, $1,$2,$3. Overall, the match failed, but the first part ($1) did match and change $1 although the rest of the expression $2,$3 didn't match and didn't change. This behavior was version specific. So, this is a cautionary note that despite what the doc says, there may be some "bite you" case out there that my super-search foo is not able to find.