#!/usr/bin/perl

use strict;
use warnings;
my ($start, $stop) = (qr '#ifdef|#if defined', qr '#endif');


open INPUT, '<', 'monks23.dat' or die "Error opening input: " . $!;
while (<INPUT>) {
    if (/$start/ .. /$stop/) {
        # Print the lines themselves
        if (/$start/ or /$stop/) { print; }
        # Skip the rest
        next;
    }
    print;
}

close INPUT;