use v5.10;

my $input;

local $" = ", ";

my $parser = qr{
    (?{
         say "before:\n\@- = (@-)\t\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
    })
    ^
    ((?&expr))
    ((?&expr))
    \z
    (?{
         say "after:\n\@- = (@-)\t\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
    })

    (?(DEFINE)
        (?<expr>
            (.) (.)
            (?{
                 say "expr:\n\@- = (@-)\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
            })
        )
    )
}x;

$input = "abcd";
chomp($input);
if ($input =~ $parser) {
  say "matches: ($&)";
  say "At the very end:\n\@- = (@-)\t ".scalar(@-)." items\n\@+ = (@+)\t ".scalar(@+)." items\n";
}
__END__