It's entirely possible that I'm way off on this, and it's sort of a Ubuntu/Linux topic as opposed to Perl but here goes my stab at helping you along...

It seems to me that USB drives are demand-mounted by Ubuntu's desktop environment (the Gnome / Unity environment), and not fstab settings. My experience has been that the drives don't get mounted until I literally click on them from the Unity desktop. They do exist, however, in /dev/*, and as such, you can mount them manually, or even create an fstab entry.

It's possible your script needs to mount the drive for you rather than waiting for Gnome/Unity to get around to it.

If the USB drive uses a FAT based filesystem, you may need to mount with cifs as well.

Again, this is based on limited experience; I haven't fiddled more than a few minutes with plugging USB drives into my Ubuntu box, and that was not recently either. But to the best of my recollection, those were obstacles I discovered, and either using the GUI to demand-mount the drive, or manually mounting it were the solutions I used.

I don't really like having things mounted in /media anyway. It just doesn't feel linux-ish. But that's personal preference, and the good thing about mounting it yourself manually is you get to follow your own whim. I put them in /mnt/*. My NAS is set up to mount in /mnt/lsxhl (Linkstation XHL), for example.

Update: The following link discusses setting up auto-mount, as well as manually mounting USB drives. It does look like automounting of USB drives is controlled by nautilus. https://help.ubuntu.com/community/Mount/USB