Ok. Well, to tell the whole story I only used DB_File in this simple example.
The thing is that I already have a db which I know has an environnement in it (I've already opened it with an other perl program). What I wanted to do was to retrieve some data from its Env, so I wanted to explore a bit the structure of an BerkeleyDB::Env class.
So I tried to create a "detached" env:
my $env = new BerkeleyDB::Env -Home => "/tmp"
or die "could not create env";
And then I do raise the "could not create" exception.
Shouldn't it be possible to create a brand new environnement, detached from any database?