Putting aside the question of what an "undefined hash" might be, what you might do is return undef on failure and a reference to a hash if all goes well. Aside from being easier to deal with, this will be more efficient since the hash contents won't get flattened into a LIST and copied back into the destination hash.

sub getHash {
  ...
  if( $hunky_dory ) {
    return \%ret_value;
  } else {
    return undef
  }
}

my $hash_ref = getHash( );

unless( defined $hash_ref ) {
  die "Woe is me\n";
}

# ... get on with it ...
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