use IO::Select;
{
  # Actually, I should have one buffer per filehandle, but
  # for simplicity, I'll assume you only ever call
  # this function with one filehandle.
  my $buffer = "";

  # Note that timeout is for waiting for the \n after the
  # first character arrives.  This function will block
  # forever waiting for the first character of the line
  sub readWithTimeout
  {
    my ($inputh, $timeout) = @_;
    if (length($buffer)==0) {
      sysread $inputh,$buffer,500;
      if (length($buffer)==0) {return undef;}
    }
    if ($buffer =~ s{^(.*\n)}{}) {return $1;}
    my $s = IO::Select->new();
    $s->add($inputh);
    my @ready = $s->can_read($timeout);
    while (@ready)
    {
      last unless
        sysread $inputh,$buffer,500,length($buffer);
      if ($buffer =~ s{^(.*\n)}{}) {return $1;}
      @ready = $s->can_read($timeout);
    }
    $buffer =~ s/.+// or return undef;
    return $&;
  }
}