I'm not sure if this is documented anywhere, or is an inadvertant feature, but the following two lines of code:
open FH, "<filedoesnotexist" or warn "$!\n";
open FH, "<filedoesnotexist" or warn "$!";
produce different output. Line one merely prints the standard error ('No such file or directory') whilst Line two prints the full "No such file or directory at C:\WIN98\DESKTOP\TEST.PL line 2."
Is this a undocumented feature (I looked at warn and die in perlfunc as well as $! in perlvar. Oh, and it's windows, so YMMV.