Thanks for the help! The bug report contained a hint that led to a trivial workaround. Namely, that %- functions correctly, while %+ has problems. From %- you can implement %+ with something like this: 
my %plus = ();
while (my ($k, $v) = each %-) {
    $plus{$k} = $v->[0] if scalar(@$v)>=1;
}

In reply to Re^2: Bug with nested named capture buffers by matthewshark
in thread Bug with nested named capture buffers by matthewshark

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