Both of these work:

print "$a[$_*3+2] " for (0..$#a/3);
print join ' ', @a[map {$_*3+2} (0..$#a/3)];
[download]

Update: Actually if @a has 3N+2 elements, the above two lines will attempt to grab an extra element from beyond the end of the list.... here are the fixed versions:

print "$a[$_*3-1] " for (1..@a/3); print "\n";
print join ' ', @a[map {$_*3-1} (1..@a/3)]; print "\n";
[download]

-Blake