pl@nereida:~/Lperltesting$ cat ./offsetsin5_10.pl
#!/usr/local/lib/perl/5.10.1/bin//perl5.10.1
use v5.10;

my $input;

local $" = ", ";

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

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

$input = <>;
chomp($input);
if ($input =~ $parser) {
  say "matches: ($&)";

##</code><code>##

pl@nereida:~/Lperltesting$ ./offsetsin5_10.pl
abab
expr:
@- = (0, , , , 0, 1)     6 items
@+ = (2, , , , 1, 2)     6 items

expr:
@- = (0, 0, , , 2, 3)    6 items
@+ = (4, 2, , , 3, 4)    6 items

main:
@- = (0, 0, 2)           3 items
@+ = (4, 2, 4, , , )     6 items

matches: (abab)