For a fixed width $width, you want to wrap your text at that width, more precisely at a blank space just before that width. This can be achieved by a regex like this s/(.{1,$width}\s/$1\n/g.

You then want to pad the last line with blanks to that specified width and then print done.. The padding can be achieved by using the following expression: $1.(' 'x($width-length($1)))."\n" instead of the one above. This would pad each line with blanks but that should not be a problem. In order to make it work, you have to specify /e in your substitution operator.