Well, I wasn't thinking of the right-hand part of a regex because:

$problem= "[";
$str= "(input)";
$str =~ s/input/input value="$problem"/;
print "<$str>\n";
# produces <(input value="[")>
[download]

So I don't see how your original problem matches this problem.

I was thinking of the fact that people surprisingly often write $str =~ /\Q$sub\E/ instead of index($str,$sub).

Now that you give us more information, it looks to me like you need to be using CGI.pm which will know how to escape your parameter values properly.

        - tye (but my friends call me "Tye")