You could use a classic C-style loop, and array slicing:

my @a=(0..12); # Admittedly, not the best choice for variable names...
my @b;

for (my $i=0; $i <= $#a; $i+=5) { 
   push @b,join(',', @a[$i..$i+4]);
   print qq[ $i:$a[$i]=@b  \n];
}
# @b ends up with  comma-separated, 5-element (or less for last chunk)
+ chunks of @a
[download]

Update: This is NON-Destructive on @a, compared to the ikegami's solution above. Not that there is anything wrong with being destructive, if you know what you are doing.