See Avoiding race condition with sysopen and the 15 odd line temp file function presented their which is the distilled guts of File::Temp (1800 lines). It also presents some args for not using File::Temp - provided you know what you are doing. Just using $$ or functions you roll yourself (unless you are an uber hacker) is quite possibly a recipe for disaster.

$$ - see perldoc perlvar

hostname is not a builtin - see perldoc -q hostname

the combination of hostname and process id is however not a safe way to generate unique strings in general:
One process will not change it's process id, but some former process will have had the same id before. Also a potential attacker could reasonably succesfully guess a future process id and create/manipulate such a file.

Edit:
There's this module on CPAN: File::Temp return name and handle of a temporary file safely

Sören

Ok, I know I run the risk of looking like a dumbass here but (assuming a Unix platform) what's wrong with $$ . time()? Is the risk that so many processes are created within one second that the PIDs roll over?

Stick a long random number on the end and your odds are pretty good. Of course, there's no reason not to look for a name collision to guarantee it.

  use Data::UUID;
  my $uuid_gen = Data::UUID->new();
  ...
  my $unique = $uuid_gen->create_str();
[download]

The generated strings are 68 characters long, containing letters, numbers and hyphens, and essentially guaranteed to be unique.