understanding code

[Nilay Puri, Noida]

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,


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