To fully exploit the marvelous utility of the unix file system naming conventions, how about this?

my $max_num = 255;
my @chars = ('a' .. 'z', 'A' .. 'Z',0 .. 9, '_'); 
my $filename = join("", map { $chars[int rand(0+@chars)] } 1 .. $max_n
+um);
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I don't have the energy right now, but a further helpful revision would be filling in all of the characters Unix allows in filenames. Oh, wait. That would be all of them except null. The mind boggles ;-)