#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11163431
use warnings;

my @what_lines;

@ARGV = '3raindeermaze.pl'; # FIXME

while( <> )
  {
  for ( split // )
    {
    /\{/ ? push @what_lines, $. :
    /\}/ ? pop @what_lines : 0;
    }
  }
print "line numbers with unmatched { => @what_lines\n";