while (@a) {
   my @group = splice(@a, 0, 3);
   ...
}

##</code><code>##

for my $i (0..@a/3-1) {
   my @group = @a[$i*3..$i*3+2];
   ...
}

##</code><code>##

for (my $i=0; $i<@a; $i+=3) {
   my @group = @a[$i..$i+2];
   ...
}

##</code><code>##

use List::MoreUtils qw( natatime );

my $i = natatime(3, @a);
while (my @group = $it->()) {
   ...
}