#!/usr/bin/perl

# https://perlmonks.org/?node_id=1218434

use strict;
use warnings;

my $string = 'THIS IS OUTSIDE (THIS IS INSIDE)';

my $inside = 0; # true if inside parens
my $onlyoutside = $string =~ s/ (\() | (\)) | ([^()]+) /
  $1 ? $inside++ x 0 : $2 ? $inside-- x 0 : $3 x !$inside /gexr;

print "before <$string>\n after <$onlyoutside>\n";
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