The second solution gets my vote, but if you are worried about the extra array, just make it a 'my' variable inside a set of braces. If you are doing this several time in your code:

sub add2hash {
   my ($value, $hashref) = @_;
   my @array = (split /X/, $value);
   $hashref->{$array[0]} = $array[1];
}
...
add2hash ('fooXbar',  \%hash);
add2hash ('bishXbash', \%hash);
[download]

OK, possibly a sledge hammer to crack a nut, depends on how many time you are doing it.
update: corrected missing $