my $str = "this is just a test\nthis is another test\nthis is yet anot
+her test\n";
$str =~ s/(.{5})/$1\n/sg;
print $str;
[download]

But that actually outputs:

---start output---
this 
is ju
st a 
test

this 
is an
other
 test

this
 is y
et an
other
 test


---end output---
[download]

Which isn't what your example showed, but is what you asked for.