Well , a .Z file is expected by some other scripts, which i do not control.
I think "compress" was chosen because gzip was not available as a standard command on some unices.

Update: Since Z compressed files can be opened by gzip, the scripts i communicate with will be able to use gunzip instead of compress , wich will allow them to read both .gz and .Z files.
And i won't have to use a system call :)

One more update: Since i also need to tar file with Archive::Tar I finally used the compression option this module provides, which creates a tar.gz . e.g :

$objTar->write($myfile.tar.gz,1) ;
[download]

Thx for your answers !