You just changed the problem massively. I didn't promise that the RE engine magically has become able to handle arbitrary parsing problems. I never said that no variables were involved. I merely said that the RE engine can handle balanced delimiters.

The two experimental features which are needed are (??{}) for delayed evaluation and (?>) for telling the engine not to backtrack. A sample script demonstrating the technique:

#! /usr/bin/perl
my $braces;
$braces = qr/(\((?:(?>[^\(\)]+)|(??{$braces}))*\))/;

while (<>) {
  if ($_ =~ $braces) {
    print "Matched '$1'\n\n";
  }
  else {
    print "No match\n\n";
  }
}
[download]

Run it and start typing in lines. The ones with balanced parentheses will match.