User-defined environment variables, like PAR_CLEAN, always overrides
the ones set in META.yml.  The algorithm for generating
caching/temporary directory is as follows:

- If PAR_GLOBAL_TEMP is specified, use it as the cache directory for
  extracted libraries, and do not clean it up after execution.

- If PAR_GLOBAL_TEMP is not set, but PAR_CLEAN is specified, set
  PAR_GLOBAL_TEMP to "TEMP/par-USER/temp-PID/", cleaning it after
  execution.

- If both are not set,  use "TEMP/par-USER/cache-HASH/" as the
  PAR_GLOBAL_TEMP, reusing any existing files inside.

  Here is a description of the variables the previous paths.

- TEMP is a temporary directory, which can be set via
  $ENV{PAR_GLOBAL_TMPDIR}, $ENV{TMPDIR}, $ENV{TEMP} or $ENV{TMP}, in
  that order of priority.  If none of those are set, C:\TEMP, /tmp
  are checked.  If neither of them exists, . is used.

- USER is the user name, or SYSTEM if none can be found.  On Win32,
  this is $Win32::LoginName.  On Unix, this is "$ENV{USERNAME}"> or
  $ENV{USER}.

- PID is the process ID.  Forked children use the parent’s PID.

- HASH is a crypto-hash of the entire par file or executable,
  calculated at creation time.  This value can be overloaded with 
  pp"’s --tempdir parameter.

By default, PAR strips POD sections from bundled modules. In case that
causes trouble, you can turn this off by setting the environment
variable "PAR_VERBATIM" to 1.