Actually, the assignment is not about Perl at all. Perl is just a convenient language to carry the assignment.

Maybe, but Perlmonks is not about doing other people's homework... but about Perl :)

The project is all about finding some vulnerabilities, in this case a race condition, and suggesting some solutions to avoid this vulnerability.

Anyway... if the OP is still here: one of the race conditions is very likely to be in the part where tmp files are created... Solution: use File::Temp.