$#content will return index of the last element in the array

OK, this explains the second half of the problem. But why do I get "the whole file" slurped into $content[0]? I had expected that each line in the file is one line in the array.

Could it have to do with line endings? The file is a Unix-Style file (\n separated), but the program is running on Windows (ActiveState Perl). But OTOH, the usual operations to read files (open() etc.) don't seem to have a problem with the different line endings...

See the Tie::File documentation on recsep:

By default, the meaning is the same as for the <...> operator: It's a string terminated by $/, which is probably "\n". (Minor exception: on DOS and Win32 systems, a 'record' is a string terminated by "\r\n".)

So that's exactly your problem. Change recsep to \n and likely everything will "just work".

Change recsep to \n

I hope this won't be necessary, because I don't know in advance which line ending style the file will have. There is, however, something I don't quite understand: As you correctly say, the documentation explains By default, the meaning is the same as for the <...> operator. But now look at the following program, which uses the angle operator insead of tie, to read the same file:

use strict;
use warnings;

sub printfirst {
  my $f=shift;
  open(F,$f) or die "$!";
  my $firstline=<F>;
  print "$f: $firstline\n";
}
printfirst('ALSF.pm');
[download]

In this case, $firstline correctly gets only the first line in the file, although the lines in the file are terminated by \n and we are running on Windows. Shouldn't then $firstline also contain the whole file in this case?
-- 
Ronald Fischer <ynnor@mm.st>