What kind of improvement are you looking for? That looks like a perfectly fine solution. How could it be any simpler?

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

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

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

use List::MoreUtils qw( natatime );

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