use strict;
use warnings;

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

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

        (?(DEFINE)
            (?<expr>
                (?{ [] })
                (.) (?{ [ @{ $^R }, $^N ] })
                #(?&foo)
                #(?&bar)
                (.) (?{ [ @{ $^R }, $^N ] })
                (?{ local @stack = ( @stack, join '|', @{ $^R } ); })
            )

            (?<foo>           )
            (?<bar> (?{ [] }) )
        )
    }x;

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

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