# MAIN
unless ($result = my_sub($val1, $val2)) {
   my $err_str = $@;
   # handle error based on $err_str
}
# do wonderful things with $result

sub my_sub {
   my ($param1, $param2) = @_;
   # do stuff
   if ($bad_data) {
      eval {die "bad data"};  # ???
      return undef;
   }
   # do more stuff
   if ($useless_param2) {
      eval {die "useless param 2"};  # ???
      return undef;
   }
   # finish doing stuff
   return $useful_result;
}