use strict;
use warnings;
my $string = '^a{4}b{2}';
my $patt = qr/$string/;
my %hsh = ($string => $string, $patt => $patt,);

my $test = 'aaaabb';

foreach (keys %hsh){
  print "YES for regex : $_\n" if $test =~ /$_/;
}
print "YES for existance : $_\n" if (exists$hsh{$test});