$hash{$italian}[1] = $french;

##</code><code>##

# Italian -> Spanish input file with 1 & 3
uno = uno 
tre = tres

# Italian -> French input file with 2 & 3
due = deux
tre = trois 

# the hash will be:
$hash = ( uno => [ 'uno' ],
          due => [ undef, 'deux' ],
          tre => [ 'tres','trois' ],
);

##</code><code>##

for my $key (keys %hash){
  if( $hash{$key}[0] and $hash{$key}[1] ){  # both are present
    print $out "$key => ", join( ' , ', @{$hash{$key}}), "\n";
  } else {                                   # one is missing
    print $out1 "$key => ", join( ' , ', @{$hash{$key}}), "\n";
  }
}