[perl #127403] [BUG] [WEIRD] Matching over 256 items yields wrong results, sometimes

[Zoffix Znet via RT]

I went to write a test for this, but can't repoduce the issue. I tried a 
handful of releases with the IRC bot, then built 2016.01 release and the 
mentioned 86a90be commit, but none of them fail.

Perhaps this is an OSX-only issue? Could someone with OSX try reproing it on 
86a90be commit and if the issue is there, would you write a test to cover the 
bug?

This is the script I was using:

    cpan@perlbuild2/tmp/tmp.uhEtludGhZ ((detached from 86a90be))$ cat foo.p6 
    my $errors = 5;        # number of errors is configurable!
    my @stuff = flat ("abc" xx 255) Z ("12" xx 128+$errors);
    
    for @stuff -> $line {
        unless $line ~~ m/1/ {
            say "seen: $line" if $line.index("2");
        }
        if $line ~~ m/9/ {
        }
    }