Do you mean to use $io->autoflush(1) ? Try this code:

use strict; use warnings;
use IO::Handle;

open my $WHOIS, "whois x.x.x.x 2>&1 |";
my $io = IO::Handle->new;
$io->fdopen($WHOIS, "r");
$io->autoflush(1);
while(my $line = $io->getline){
    print $line;
}
$io->close;
[download]

This is working fine on my environment.