You could log the IP addresses to a file and then search it. If you're running your site with a database you could log them to a table.

$address=$ENV{REMOTE_ADDR};
open FILE, "<logaddresses.dat";
my $addresses;
{
local $/=undef;
$addresses=<FILE>;
}
close FILE;
if($addresses=~/\Q$address/){
   print "You already voted\n";
} else{
   open FILE, ">>logaddresses.dat";
   print " $address\n";
   close FILE;
   # update poll;
}
#display results;
[download]

if you want code for a database solution let me know