Something like this? (UNTESTED):

my $count = 0;
my $outfile = 'A';
open DISTLISTS, '<', 'rdy.csv' 
    or die "Unable to host file for splitting: $!\n";
while my $dist_split ( <DISTLISTS> ) {
    if( $count % 200 == 0 ) {
        close OUTFILE unless $count == 0;
        open OUTFILE, '>', $outfile . '.csv'
            or die "Couldn't open output file: $!\n";
        $outfile++;
    }
    print OUTFILE $dist_split;
    $count++;
}
close OUTFILE or die "Bleah! $!\n";
close DISTLISTS;
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I haven't tested this, but it should get you going down a road that will lead to a workable solution.