I found a solution that adds 4 characters per capture:

use strict;
use warnings;

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

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

        (?(DEFINE)
            (?<expr>
                (?<i> . )
                (?<i> . )
                (?{ local @stack = ( @stack, $-{i}[0] . '|' . $-{i}[1]
+ ); })
            )
        )
    }x;

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

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

or

use strict;
use warnings;

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

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

        (?(DEFINE)
            (?<expr>
                (?<i> . )
                (?<j> . )
                (?{ local @stack = ( @stack, $+{i} . '|' . $+{j} ); })
            )
        )
    }x;

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

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