$original = &promptUser("Enter original filename");
$modified = &promptUser("Enter modified filename");

open($ORIG, "$original");
binmode($ORIG);

open($MOD, ">$modified");
binmode($MOD);

$/ = "\x00";

while (<$ORIG>) {

$count = (($change = $_) =~ s/(\x33\x33\x39\x39.*?\x00)/"\x00" x length($1)/e);

if ($count == null) {

print $MOD $_;

} else {

	$answer = &promptUser("Modify $1?");
	if ($answer == "y") {
	print $MOD $change;
	} else {
	print $MOD $_;
	}

}
}

close $MOD;
close $ORIG;

sub promptUser {

   local($promptString,$defaultValue) = @_;

   if ($defaultValue) {
      print $promptString, "[", $defaultValue, "]: ";
   } else {
      print $promptString, ": ";
   }

   $| = 1;               # force a flush after print
   $_ = <STDIN>;         # get the input from STDIN

   chomp;

   if ("$defaultValue") {
      return $_ ? $_ : $defaultValue;    # return $_ if it has a value
   } else {
      return $_;
   }
}