"I wonder if printing out backspaces isn't the most efficient way to erase the line."

Actually, "\r" serves that purpose, unless you are overprinting in mid-line.   But even then, simply reprinting the unchanging beginning of the line still seems simplest.
eg,

printf(" %3d %s\r", $time, $line);

while(--$time) {
  sleep 1;
  printf(" %3d\r", $time);
}
print"\n";
[download]

See this whirligig for a similar function.

  p