my @all;
my @group1_s = sort compare_func @group1;
my @group2_s = sort compare_func @group2;
while (@group1_s && @group2_s) {
   my $cmp = compare_func($group1_s[0], $group2_s[0]);
   if    ($cmp < 0) { push(@all, shift(@group1_s)); }
   elsif ($cmp > 0) { push(@all, shift(@group2_s)); }
   else             { push(@all, scalar(shift(@group1_s), shift(@group2_s))); }
}
push(@all, @group1_s, @group2_s);