sub solution_2 {
    # adaptive motion queue solution
    my ($list_ref1, $list_ref2) = @_;
    my @list1;
    my @list2;
    if (@$list_ref1 <= @$list_ref2) {
       @list1 = @$list_ref1;
       @list2 = @$list_ref2;
    } else {
       @list1 = @$list_ref2;
       @list2 = @$list_ref1;
    }
    
    my @queue = map $_+$list2[0], @list1;
    my @xs = (0 .. $#list1);
    my @ys = (0) x @list1;
    
    while (@queue) {
        print OUT shift(@queue), "\n";
        my $x = shift @xs;
        my $y = shift @ys;
        next if ++$y >= @list2;
        
        my $sum = $list1[$x] + $list2[$y];
        
        my $count = 0;
        $count++ until $count == @queue or $sum <= $queue[$count];
        splice @queue, $count, 0, $sum;
        splice @xs, $count, 0, $x;
        splice @ys, $count, 0, $y;
    }
}

sub solution_3 {
    # adaptive motion queue solution, one array
    my ($list_ref1, $list_ref2) = @_;
    my @list1;
    my @list2;
    if (@$list_ref1 <= @$list_ref2) {
       @list1 = @$list_ref1;
       @list2 = @$list_ref2;
    } else {
       @list1 = @$list_ref2;
       @list2 = @$list_ref1;
    }
    
    my @queue = ();
    for (0 .. $#list1) {
        push @queue, join("x", $list1[$_]+$list2[0], "$_", "0");
    }
    
    while (@queue) {
        my $entry = shift(@queue);
        my ($old, $x, $y) = split /x/, $entry;
        print OUT "$old\n";

        next if ++$y >= @list2;
        
        my $sum = $list1[$x] + $list2[$y];
        
        my $count = 0;
        {
            no warnings 'numeric';
            $count++ until $count == @queue or $sum <= $queue[$count];
        }
        splice @queue, $count, 0, "${sum}x${x}x${y}";
    }
}

##</code><code>##

Benchmark: timing 100 iterations of 1_array, 3_array, Baseline, LR_1...
   1_array: 10.651 wallclock secs (10.65 usr +  0.00 sys = 10.65 CPU) @  9.39/s (n=100)
   3_array: 9.28456 wallclock secs ( 9.28 usr +  0.00 sys =  9.28 CPU) @ 10.78/s (n=100)
  Baseline: 0.844141 wallclock secs ( 0.84 usr +  0.00 sys =  0.84 CPU) @ 119.05/s (n=100)
            (warning: too few iterations for a reliable count)
      LR_1: 20.4537 wallclock secs (20.45 usr +  0.00 sys = 20.45 CPU) @  4.89/s (n=100)
           Rate     LR_1  1_array  3_array Baseline
LR_1     4.89/s       --     -48%     -55%     -96%
1_array  9.39/s      92%       --     -13%     -92%
3_array  10.8/s     120%      15%       --     -91%
Baseline  119/s    2335%    1168%    1005%       --

Benchmark: timing 100000 iterations of 1_array, 3_array, Baseline, LR_1...
   1_array: 7.06968 wallclock secs ( 7.07 usr +  0.00 sys =  7.07 CPU) @ 14144.27/s (n=100000)
   3_array: 5.09259 wallclock secs ( 5.09 usr +  0.00 sys =  5.09 CPU) @ 19646.37/s (n=100000)
  Baseline: 2.11864 wallclock secs ( 2.12 usr +  0.00 sys =  2.12 CPU) @ 47169.81/s (n=100000)
      LR_1: 7.01637 wallclock secs ( 7.02 usr +  0.00 sys =  7.02 CPU) @ 14245.01/s (n=100000)
            Rate  1_array     LR_1  3_array Baseline
1_array  14144/s       --      -1%     -28%     -70%
LR_1     14245/s       1%       --     -27%     -70%
3_array  19646/s      39%      38%       --     -58%
Baseline 47170/s     233%     231%     140%       --