# by assigning directly to the second element of the sub-array
$hash{$italian}[1] = $french;

# or by dereferencing the sub-array pointed to by the hash value
# and pushing the new value onto the end of that array
push @{$hash{$italian}}, $french;

# Either way, you'll end up with:
$hash = ( uno => [ 'uno','un' ],
          due => [ 'dos','deux' ],
          # and so on
         );