List::MoreUtils::XS::pairwise allows variables to pollute into next iteration

[bleargh45]

While testing updates to CPAN modules recently, I ran across a case where variables can persist and pollute across into the next iteration of a pairwise { ... } evaluation.

The following one-liner shows this pollution:

$ perl -MList::MoreUtils=pairwise -e '@let=qw(a b c); @num=qw(1 2 3); pairwise { my $out; $out.=$a; print "$out\n" } @let, @num'
a
ab
abc

When run with just the PP code, however, it works as intended:

$ LIST_MOREUTILS_PP=1 perl -MList::MoreUtils=pairwise -e '@let=qw(a b c); @num=qw(1 2 3); pairwise { my $out; $out.=$a; print "$out\n" } @let, @num'
a
b
c

Going back through past releases, I was able to trace this issue back to between 0.418 and 0.420_001; 0.418 works as intended, but 0.420_001 and all subsequent releases allow the value to pollute into the next iteration.