many thanks, it is solved,
If combine BrowserUk's and mwah's solutions, then it works fine

$x=~s/(.{1,3}|^$)\n?/sprintf"'%-3s'\n",$1/eg;
[download]

it works even on multiline strings
---
and even multiline with emptyline mixed together

$x=~s/(.{1,3}|^$)\n?/sprintf"'%-3s'\n",$1/meg;
[download]