$line = "x" x 38 . "\n";

sub test {
  ($res) = ($_[0] =~ /((?:[^\n]{0,40}\n){0,4})/);
  if ($res) {
     print "yes\n";
  }
  else {
     print "no\n";
  }
}

test( "x$line" x 4 );    # 39 x 4
test( "xx$line" x 4 );   # 40 x 4
test( "xxx$line" x 4 );  # 41 x 4
test( "xx$line" x 3 );   # 40 x 3
test( "x$line" x 5 );    # 39 x 5