sub between {
    my ($text) = @_;
    my $level = 0;
    my @comments = ();

    while ( $text =~ m{\G .*? (start|end) (.*?) (?: (?=start|end) | $ ) }gxs ) {
        if ( $1 eq 'start') { $level++; }
        else { ($level > 0) and $level--; }

        $level > 0 and push(@comments, $2);
    }

    return join('', @comments);
}