If you don't want to destroy the array with splice:

[0] Perl> sub do3{ print "@_" };;
[0] Perl> @a = 1 .. 20;;
[0] Perl> do3 @a[ $_ .. $_ +2 ] for map{ $_ * 3 } 0 .. $#a / 3;;
1 2 3
4 5 6
7 8 9
10 11 12
13 14 15
16 17 18
Use of uninitialized value in join or string at (eval 3) line 1, <STDI
+N> line 3.
19 20
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If your array might not be a multiple of 3, then you'll need to take appropriate precautions inside the sub.