I prefer

{
   open local *STDOUT, '>', \$buf;
   ...
}
[download]

open TEMP, '>', \$buf;
my $old_select = select(TEMP);
...
select($old_select);
[download]

since the former restores STDOUT even in the case of exceptions. The former, however, doesn't work if someone has previously called select. I guess the comprehensive solution would be:

{
   my $old_select = select();
   my $handle = on_release { select($old_select); };
   open local *STDOUT, '>', \$buf;
   select(STDOUT);
   ...
}
[download]

It even handles exceptions thrown by signal handlers.

Updated.