This looks fishy:
my $exec = "/usr/local/bin/monster -i$id $filePath$file &";
qx/$exec/;
Calling exec in scalar context like this, you're feeding what appears to be user input to /bin/sh. If an external user controls the value of $id, $filePath, or $file, you'll get owned because the shell interprets metacharacters. E.g. if $id is set to `rm -rf /`, sh will execute it.
Also, I'm not sure if this is related, but there's no need for the '&' at the end of the command. You've already forked a child, so you shouldn't need to fork again. Try this:
exec("/usr/local/bin/monster", "-i", $id, "$filePath$file");
Calling exec() in array context executes the program directly rather than feeding it to the system shell, so metacharacters won't be a problem.
-Matt