You don't need a slice if you're sorting the array reference members of @sortarr by a particular set of elements. Assuming numeric values, hence <=> comparisons,

@sortarr = sort { $a->[3] <=> $b->[3] } @sortarr;
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If you want to sort on several columns, test in your desired order with logical-or between,

@sortarr = sort {
    $a->[6] <=> $b->[6]
    ||
    $a->[7] <=> $b->[7]
    ||
    $a->[8] <=> $b->[8]
    ||
    $a->[9] <=> $b->[9]
    ||
    $a->[10] <=> $b->[10]
    ||
    $a->[11] <=> $b->[11];
} @sortarr;
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Most of that is rarely executed because of the short circuit property of logical or. Looking up values by index is cheaper than copying out a 2-d slice sorting it and correlating the result to the original.

If you do ever need slicing and other ops on deep arrays, see the great PDL module.