You can't rely on the commands making it through in one piece.

Furthermore, there are recommendations to avoid using buffered IO (<SOCK>, read, eof, getline, getlines) in the docs. sysread is not buffered.

• For each socket with data available (as returned by select),
  • Read (using sysread) from the socket.
  • If sysread returned undef,
    • Remove socket from the select object.
    • Handle error socket.
  • If sysread returned zero,
    • Remove socket from the select object.
    • Handle closed socket.
  • Append to the socket's buffer the data that was read from the socket.
  • While the buffer has a full line/command in it,
    • Extract the line/command from the buffer (leaving the rest in the buffer).
    • Process the extracted line/command.

my ($r_ready, $w_ready) = IO::Select::select($r_sel, $w_sel);

foreach my $sock (@$r_ready) {
   # Create alias to the appropriate buffer.
   our $buf; local *buf = \$in_bufs{$sock};

   my $rv = sysread($sock, $buf, 4096, length($buf));

   # Read error.
   if (not defined $rv) {
      $r_sel->remove($sock);
      # ... Handle error socket ...
      next;
   }

   # Socket EOF
   if (not $rv) {
      $r_sel->remove($sock);
      # ... Handle closed socket ...
      next;
   }

   for (;;) {
      # Search for a complete command.
      my $pos = index($buf, "\x0A");
      last if $pos < 0;

      my $cmd = substr($buf, 0, $pos+1, '');
      # ... process $cmd ...
   }
}

foreach my $sock (@$w_ready) {
   # ...
}
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Update: Added code.
Update: Bug fix to handle the case where multiple commands in the buffer.