my @a = sort { $a <=> $b } (43,43,44);
my @b = sort { $a <=> $b } (43,43);

my @c;
while (@a && @b) {
   if ($a[0] < $b[0]) {
      push @c, shift @a;
   }
   elsif ($a[0] > $b[0]) {
      die "Bad data";
   }
   else {
      shift @a;
      shift @b;
   }
}

push @c, $_ for @a;
die "Bad data" if @b;