If you're a hardcore regexp junkie, you might prefer to ^[]$\\?. it:

sub cut {
  my ($str, $n) = @_;
  ($str =~ /^(?=.{$n})(.{0,@{[$n-1]}})\s(.*)$/s, $str, '')[0,1];
}
[download]

Greed helps here, of course. It always returns both halves of the string, though.