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;

##</code><code>##

a|b
c|d

##</code><code>##

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

            (?<foo>
                .
            )