for commoners, like myself. my problem is easier to show than to explain (perl 5.8.8):

#!/usr/bin/perl -w
use strict;
use Regexp::Common;

my $teststring= "teststring: start hello {ab}{cd} end\n";

my $balancedparens= qr/\s*$RE{balanced}{-parens=>'{ }'}/;

my $pattern1= $balancedparens . $balancedparens;
my $pattern2= qr/hello/;
my $pattern3= $pattern2.$pattern1;

print "no error yet\n";

$teststring=~ s/$pattern3/hi$1/g;  ## ERROR, WHY???

print $teststring;
[download]

can someone please explain to me how to avoid this (and not just suppress the error)?