Something else isn't right then. Using the data you supplied above (which you should copy to your actual question for future readers), and copy/pasting Athanasius' regex, here's what I get in %errors:

use warnings;
use strict;

my $string = "Return code is";

my %errors;

while (<DATA>){
    ++$errors{$1} if /\Q$string\E \s* : \s* (\d+) $ /x;
}

while (my ($k, $v) = each %errors){
   print "$k: $v\n";
}
__DATA__
2015/04/06 12:38:15 (9):(5564) Return code is :55
2015/04/06 12:38:15 (9):(6600) Return code is :55
2015/04/06 12:41:31 (9):(3604) Return code is :1
[download]

Output:

1: 1
55: 2
[download]