I'm running alien in a directory where I have just unpacked a source tar.gz file
and made it into deb. This means the directory <program-name>-<source-version>
exists. As in this simulation:

fettuce$ mkdir paa-1
fettuce$ alien -r paa_1-2_all.deb
Warning: alien is not running as root!
Warning: Ownerships of files in the generated packages will probably be wrong.
mkdir: cannot create directory `paa-1': File exists
unable to mkdir paa-1:  at /usr/share/perl5/Alien/Package.pm line 257.
fettuce$

Without my mkdir command it runs correctly.

Obviously, it is not for the user to "second guess" what name alien is going
to want to use for its work directory. Perhaps alien could use mktemp to
generate the name of the work directory or maybe use mktemp to make a sandbox
in which alien could use any name it wanted?

Let me know if you need more info! Thanks and greetings

Alexis