If your input is a fixed field record, then unpack function could really make you smile :-) like so:

#!/usr/bin/perl
use warnings;
use strict;

my $str =
  'HWI-1KL120:99:C0C9MACXX:6:1101:2105:2123 0 chr5 75483987 0 82M3I16M
+ * 0 0';

my ( $fifth, $sixth ) = unpack "x31A4xA4", $str;

print $fifth, ":", $sixth, $/; # prints 2105:2123

#or even better

my ($str_wanted) = unpack "x31A9", $str; 

print $str_wanted, $/;        # prints 2105:2123
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