if (defined($hash{$key}) {
    $hash{$key} ++;
  } else {
    $hash{$key} = 1;
  }
[download]

That's actually more work than necessary.
All you need is     $hash{$key}++; because if the element doesn't yet exist, it will be created and initialized ("autovivified") to undef, then the undef will be nummified to 0, then the 0 will be incremented to 1.

  %hash = ("www.yahoo.com", 10, "www.google.com", 2000);
  foreach (keys %hash) {
    $rev_hash{$hash{$_}} = $_;
  }
  foreach (sort keys %rev_hash) {
    print "$rev_hash{$_} appears: $_ time(s)\n";
  }
[download]

No, that's bad. What if both yahoo and google have a value of 100?
All you're really trying to do it print out the entries sorted by value, numerically.
(And, btw, you were sorting lexically, which is also wrong.)

  %hash = (
     www.yahoo.com => 10,
     www.google.com => 2000,
  );

  for ( sort { $hash{$a} <=> $hash{$b} } keys %hash )
  {
    print "$_ appears: $hash{$_} time(s)\n";
  }
[download]

jdporter
...porque es dificil estar guapo y blanco.