use IO::Select qw( );

# I like big blocks and and I can not lie.
# Benchmarking might just deny.
use constant BLK_SIZE => 64*1024;

my $sel = IO::Select->new($fh);

my $buf = '';
while ($sel->can_read()) {
   my $rv = sysread($fh, $buf, BLK_SIZE, length($buf));

   if (!defined($rv)) {
      ... Handle error. Don't forget $buf might not be empty. ...
      $sel->remove($fh);
   }

   if (!$rv) {
      ... Handle EOF. Don't forget $buf might not be empty. ...
      $sel->remove($fh);
   }

   while ($buf =~ s/^(.*)\n//) {
      my $msg = $1;
      my $close = process_msg($msg);
      $sel->remove($fh) if $close;
   }
}