The 'die' would go after the foreach (refer to the pseudocode that I posted). Note the comments below:

sub get_family_member {
  my @args = @_;
  my $who = shift @args; # <-- this var is not used
  my $families = shift @args;
  foreach my $family (%families) {
    if ($families->{$family}->{kid} eq "bart") { # <-- are you sure yo
+u want to hard-code 'bart'? I think you need $who
      return $dfamilies->{$family}->{kid}; # <-- typo in hash name
      last; # <-- this will never be reached since it already returned
    }
  }

  # no matches found:  die here
}
[download]