the obligitory mention of Taint mode. you should add a -T to the /usr/bin/perl -w line. Then you need to check the validity of the input line from the web page. you are trsting the user not to put extra .. and /'s in that could redirect the open command to operate on say ../../../etc/passwd for example.
You are making assumptions that the filename given will be file.ext and it will break when it gets something like file.ext.gz which is valid on some OSs
You could save yourself some parseing of the filename by using
FILE::BASENAME
UPDATE:Fixed 'not enough sleep this week' typo picked up by blakem