You might be able to use Win32::API and SetVolumeMountPoint to do this. There is a little more information here and some sample C code here.

You might also get away with shelling out to the subst command

P:\test>help subst
Associates a path with a drive letter.

SUBST [drive1: [drive2:]path]
SUBST drive1: /D

  drive1:        Specifies a virtual drive to which you want to assign
+ a path.
  [drive2:]path  Specifies a physical drive and path you want to assig
+n to
                 a virtual drive.
  /D             Deletes a substituted (virtual) drive.

Type SUBST with no parameters to display a list of current virtual dri
+ves.
[download]

which might be simpler, but would require you to detect where the USB drive was so that you could subst it to a well-known place.