$x = "piyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyush";
$x =~ s#(.{64})#$1\n#gsi;
print $x;

o/p: 
piyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyushpiyu
shpiyushpiyushpiyushpiyush