Of course I tried a

my $str = "Hello";
print reverse $str;
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Nothing happened. Scalar context scalar context... $str is a scalar no? ;-)

Then I had a look at the Perl Cookbook and the solution "processing a string one character at a time" involved the creation of a list, which isn't acceptable because I have to reverse whole files (can do so in memory, but not with the file in a scalar AND splitted each byte in a list).

I then ended up with a solution that worked but instantly didn't feel perlish:

sub rev_scalar {
  my $data = shift;
  my $i;
  my $tmp;

  for($i = 0; $i < int(length($$data)/2); $i++) {
    $tmp = substr($$data,$i,1);
    substr($$data,$i,1) = substr($$data,-($i+1),1);
    substr($$data,-($i+1),1) = $tmp;
  }
}
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Ugly isn't it. Simple things should be simple. Therefore there MUST be a better solution in Perl.

reverse is this solution, but I didn't see it because I tested it within print which - I suppose - puts the whole thing into list context.

Ah well. So a  print scalar(reverse $str); should work. And it does. And now is time to admit, that whatever skills I may have reached in Perl, I still haven't sufficient intuition about the list/scalar context thing.