I believe Active State Perl on Windows implements the POSIX module -- http://aspn.activestate.com/ASPN/docs/ActivePerl/5.8/lib/POSIX.html -- so maybe:
use POSIX qw(tmpnam);
my $tmp_name = tmpnam();
In general, I think it's a laudable goal to write code that's 100% portable, and works properly on all platforms without any special casing. Realistically, I don't think that's possible for any perl program of any significant complexity. Maybe it's doable if everything your students are writing is just short exercises. The one time I wrote a cross-platform perl app to run on both Windows and Unix, all I managed to do was isolate all the OS-dependent stuff in one file, but I did have to have a bunch of special casing in that file. I remember lots of hassles with, e.g., filename globbing.