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...
sub groupsOf (&$@);
...
    &$rcDoIt;
}
...

use strict;
use warnings;
...
    push @arr, shift @arr for 1 .. $places;
    return @arr;
}

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2 1 4 3 6 5 8 7 10 9 12 11 14 13 16 15 18 17 20 19
...
4 5 1 2 3 9 10 6 7 8 14 15 11 12 13 19 20 16 17 18
----------------------------------------
21 57 93 39