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