My first approach wouldn't be to do this with a single regexp. Instead, consider the following:

• If the string length is <= 45 characters, it wins
• Otherwise, extract the first 46 characters, and apply $str =~ s/\s\S*?$//; which trims the string back to a word boundary. This doesn't guarantee the that resulting string will be 45 characters or less, though, since someone could provide a long string with no whitespace.

You might also consider things like first deleting leading spaces, and collapsing multiple spaces into one.