@a = 'a'..'z';;

$s = join ',', @a;;

$s =~ s[(,[^,]+){6}\K,][\n]g;;

print $s;;
a,b,c,d,e,f,g
h,i,j,k,l,m,n
o,p,q,r,s,t,u
v,w,x,y,z