From Memory :

• Newlines causes trouble (\n is different among platform and catches 1 or 2 char...)
• Case sensitivity for filenames can screw your unicity tests...
• Some filenames can cause trouble on some OSes
  Avoid CON, PRN, AUX...
  (In fact only one OS was dumb enough to crash when accessing a c:\con\con ;-)

But probably a lot more in perlport

"Only Bad Coders Code Badly In Perl" (OBC2BIP)