understanding code

From: Nilay Puri (nilayp_at_noida.hcltech.com)
Date: 01/31/04 

To: beginners@perl.org
Date: Sat, 31 Jan 2004 17:35:47 +0530

Hi all,

Can any one walk me thru this piece of code ::

while(<STDIN>)
{
        chomp ;
        $isbn =(split(/^_/, $_))[0] ; --- not able to understand what is
being accessed (......)[0]
        unless ($KEYS{$isbn} ) ---- isbn is a scalar variable, how keys
wok on it ?
        {
                print "$_\n" ;
                $KEYS{$isbn} =1 ;
        }
}

Thanks,