I like to use "for" for that kind of thing

Me too, but I have found do{} is more readable. For one thing, it saves you from your bug: Your version throws away the uppercase string and so prints just "goodbye todo el mondo". My version would look like this:

my $sBegin = "hello world";
my $sEnd;
for ($sEnd = $sBegin) {
  s/hello/goodbye/g;
  s/world/todo el mondo/;
  $_ = uc;
};

print $sEnd;
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TIMTOWTDI, but I think do{} is the more readable here.

Update: I trust you. No problem. But the bug still illustrates how the void context makes the for loop slightly less readable here. Such a bug (update: or the more likely opposite bug) could not as easily occur in the do{} block.