You're not resetting the value of $i. Try this:

open (LOG, "$fw_logfile") or die and mail_error($!);
open (OUTFILE, "+>>$outfile") or die and mail_error($!);
 
my $seen = 0;
 
while(<LOG>){
    foreach my $i (@newips){
        unless($seen == 500){
            print OUTFILE if (/\;drop;[^"]*$i/) && $seen++;
        }
    }
    $seen = 0;
}
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