use strict;
use warnings;

my $str1 = "ABCXXFGH";
my $str2 = "XXCDEFGH";

my $str3 = "XXCXXFGH";


my ($str_to_retain) = $str2 =~ /(\w\w\w).*/;
my ($replacement) = $str1 =~ /\w\w\w(.*)/;

my $replaced_str = $str_to_retain.$replacement;


print "$str3\n$replaced_str\n";