how to clip an existing pdf file

Hi:

I set up an web interface in php, so that the user clips portion of an
image (which is created by converting pdf to image, embeded on webpage)

I pass the coordinates (converted to point unit later) of the clip
box. In the backend, run some codes to clip the corresponding pdf.
Then pop up a download page to download the pdf.

I wonder how can I clip an existing pdf file, then save the portion to
let user to download.

newbie for perl.....