thanks,with:

my $jroot="";
foreach my $a (keys %INC)
  {if ($INC{$a}=~/(.*)jinit.pm/){$jroot=$1}}
die "I cannot find datadirectory\n" if ($jroot eq "");
[download]

I got what I wanted.
Still, I don't like to use the pattern "jinit.pm" (the module name itself) hardcoded inside, to find the current directory - but it's just a problem of elegance, nothing more...

alessandro