The regular expression captured submatches ($1, $2, ...) are now more consistently unset if the match fails, instead of leaving false data lying around in them.
I believe this is referring to the fact that if a match has $1..$6 in one successful match, then only $1..$2 in the next match, that $3..$6 are more carefully set to undef now. I remember there being a patch to that effect.
If they were to change the other behavior, it would no longer be (usefully) upward compatible. This would have been a grave mistake, caught by someone (like me).
The reason the variables are not unset on failure is to permit:
if ($string = /... ( ... ) .../) { # overall match
if ($1 =~ /foo/) { # and $1 contains foo
...
} elsif ($1 =~ /bar/) { # nope? how about bar
...
}
}
If $1 reset on the failed match on the second line, then we couldn't
do the third match. This has been by design in Perl for over a decade now.
(And I just verified that 5.8.5 doesn't unset the variables, so it works as it always has.)