Just found another nice solution:

$window=79;
$str="X" x 723;
print join "\n", unpack "A$window" x ((length($str)+$window-1)/$window
+), $str;
[download]

String is split by unpack into enough pieces of $window length. "A$window" is the pattern for one piece. This is multiplied by (length($str)+$window-1)/$window.
the resulting array is then joined with "\n";