Here's a similar way, without initializing either counter:

push @new, $array1[$count1++].$array2[--$count2]
    while ($count1 < @array1 && -$count2 < @array2);
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By the way, your array accesses are using 1 element slices instead of direct element access. Consider changing @array1[$a1count] to $array1[$a1count].