#/usr/bin/perl -w
use strict;

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

my ($fifth, $sixth) = (split(/\s+|:/,$in) )[5,6];

print "$fifth:$sixth\n"; #2105:2123