I would do very much the same thing, with a little tweaking:

sub sg {
   my $res;
   
   # I won't assume COND being undefined is invalid

   if (COND) {
      $res = 1;
   }
   else {
      $res = 2;
   }

   die "Undefined result" unless defined $res;
   return $res;
}
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Of course, this I'd only use this if the code were much more complex than simply assigning the result.