Try this:

...

open(TESTFILEHANDLE, "<$yourfilename") && die "$!, $yourfilename already exists.\n";

...

The && die... will kill the program and display the message, IF AND ONLY IF, the file was successfully opened (readonly), which in your case, is what you wanted. You can also replace the "die" keyword with "warn" if you don't want to kill the program.

Of course, it's a good example to close the filehandle before exiting the program.

I hope it helps.