Hi almut,
I'm sorry, this excursion was very interisting, but that *may* work from the unix side, but on my both XP systems (v5.8.8 and v5.10.0 with komodo) it doesn't work; the yield is only *one* array element.
BUT I have to apologize! The solution is written in the 'ActivePerl User Guide -- Tie::File', witch I did read some times before.
There is another parameter called 'recsep' that worked very well for me. So here is the code.

use Tie::File;        # Record separator per default = $/
use Fcntl 'O_RDONLY';   # open in read-only mode
my ($sep1, $sep2) = ($ENV{'windir'})   # determine OS
      ? ("\r\n", "\n") : ("\n", "\r\n");
my $file = shift @ARGV;
tie my @array, 'Tie::File', $file, mode => O_RDONLY,
      memory => 0, recsep => $sep1 or die $!;
my $n_recs = @array;    # how many records are in the file?
unless ($n_recs > 1) {
   untie @array;
   tie @array, 'Tie::File', $file, mode => O_RDONLY,
         memory => 0, recsep => $sep2 or die $!;
   $n_recs = @array;
   print join ("~SEP~", @array);
   die "Found only one array element\n" unless $n_recs > 1;
}
[download]

Voila! Proggy dies not (in XP).
Nevertheless thanx for your help, almut.
... And I'm still wondering, why neither changing of $/ nor the setting of "<:unix" had any effect at all.