Got a better solution for you: 
use strict;
use warnings;

sub rel_cap { my ($ofs) = @_; substr($_, $-[$ofs], $+[$ofs] - $-[$ofs]
+) }

sub parser {
    local our @stack;
    local our @rv;

    my $parser = qr{
        ^
        (?&expr)
        (?&expr)
        \z
        (?{ @rv = @stack; })

        (?(DEFINE)
            (?<expr>
                (.)
                (.)
                (?{ local @stack = (@stack,
                    rel_cap(-2) . "|" . rel_cap(-1) ); })
            )
        )
    }x;

    return $_[0] =~ /$parser/ && \@rv;
}

my $rv = parser('abcd');
print("$_\n") for @$rv;

[download]
a|b
c|d

[download]

Update: Nevermind. @- and/or @+ become all wonky if there's a (?&...) inside the (?<...>...).

Even something as simple as the following fails: 

            (?<expr>
                (?&foo)
                (.)
                (?{ local @stack = (@stack, rel_cap(-1) ); })
            )

            (?<foo>
                .
            )

[download]

In reply to Re: Backreference variables in code embedded inside Perl 5.10 regexps (try2) by ikegami
in thread Backreference variables in code embedded inside Perl 5.10 regexps by casiano

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