If you want to replace all occurances of HOST and IP in your mainfile.txt during the copy process, you just need to replace the printing part:

 ...

 foreach my $txt (@lines)
 {
   # replace HOST and IP with $hostname and $ipaddr
   $txt =~ s/HOST/$hostname/g;
   $txt =~ s/IP/$ipaddr/g;
   print $output "$txt\n";
 }
[download]

This will replace ALL occurances of HOST and IP in your mainfile.txt.