Autoflushing isn't the solution to this buffering problem.

I'll guess that getline() does buffered I/O. This means that, if two messages are ready to be read, getline() might read them both into its buffer when you call it. Then it splits off the first line and returns it to you. Then you ask select to tell you when there is more data to be read off the socket.

Well, there isn't any more data to be read off the socket. But there is more data in getline()'s buffer.

I'd probably do this more like:

my %buf;
while (1) {
  my @ready = $Select->can_read(1);
  foreach $fh ( @ready ) {
    while(  sysread( $fh, $buf{$fh}, 1024, length($buf{$fh}) )  ) {
      if(  $buf{$fh} =~ s#(.*$/)##  ) {
        my $line= $1;
        dostuffwith( $line );
      }
    }
  }
}
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