This is best done with a hash during user entry. See How do I use a hash as a set? for more details, but basically:

while(<STDIN>){
    push @array, $_;
    if($hash{$_}){
        print "$_ was entered more than once\n";
    } 
    $hash{$_}++;
}
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If your hash is declared as a my() variable inside a block during entry, you can even have the memory freed when you leave the block.