Which version of OS X? This is probably a job for so-called "Folder Actions" (10.3 Panther and later) which allow you to fire off a script in response to adding files. Then you'll need a bit of AppleScript (using "on adding folder items") that fires off your perl script (using "do shell script"). A good PerlMonks-ish site to look for this sort of thing is http://bbs.applescript.net. Good luck.