internal server error after changing perl module

while working on my development server today i made some changes to a perl module adding one new sub to it
after restarting the server to allow the changes to be picked up i now get a error when trying to access some of the pages on my web application

The server encountered an internal error or misconfiguration and was
unable to complete your request. Your administrator may not have enabled
CGI access for this directory.

i have made no changes to the access or config files. this error still occurs after i restored the backup of the files i made the changes to.

from looking at the pages that have stopped working they all seem to be the ones that use the file i changed but im sure i restored the original file so i dont really understand what i've done.

anyone have any ideas what i can try to fix it?

thanks for any help
mike