because the OP doesn't want to clobber what's already in the hash.

Maybe, but the OP proposed a solution that do reset the hash :

# Works, but needs redundent array
undef %hash;
@array = (split /X/, "fooXbar");
$hash{$array[0]} = $array[1];
@array = (split /X/, "bishXbash");
$hash{$array[0]} = $array[1];
foreach(keys %hash){
  print "2: $_ = $hash{$_}\n";
}
[download]

so what is to be achieved isn't that clear after all.