...
   print "Got: $in\n";
   if ($in =~ /#:#$/) {
     ...
   } elsif ($in =~ /_$/) {
     ...
   } elsif ($in =~ /__END$/) {
     ...
   } else {
     ...
     # save file
   }

##</code><code>##

Got: X-Call~4.xml#:#380_<?xml version="1.0"?>

##</code><code>##

if ($in =~ /__END$/) {
  # close previous file
} else {
  ($filename, $filesize) = $in =~ /^([^#]+)#:#(\d+)_/;

  if ($filename) {
     # create new file
  }

  # proceed with saving the file
}