Did you mean something like this?

Data like 1nickt but with dups.

fred,barney,wilma,betty
foo,bar,baz,qux
foo,bar,baz,qux
fred,barney,wilma,betty
fred,barney,wilma,betty
apple,orange,banana,pear
apple,orange,banana,pear
foo,bar,baz,qux
fred,barney,wilma,betty
apple,orange,banana,pear
[download]

#!/usr/bin/env perl

use strict;
use warnings;
use feature qw (say);
use Data::Dump;
use Iterator::Simple qw(iter);

my $file = q(1204245.csv);

my %column;

open my $fh, q(<), $file or die $!;

while (<$fh>) {
    chomp;
    $column{ ( split /,/ )[2] } = undef;
}

close $fh;

dd \%column;

# what ever..
say for sort { $b cmp $a } keys %column;

say q(--);

# or
my $iterator = iter( IO::File->new($file) );

while (<$iterator>) {
    chomp;
    $column{ ( split /,/ )[2] } = undef;
}

dd \%column;

say for sort { $a cmp $b } keys %column;

__END__

karls-mac-mini:monks karl$ ./1204245.pl
{ banana => undef, baz => undef, wilma => undef }
wilma
baz
banana
--
{ banana => undef, baz => undef, wilma => undef }
banana
baz
wilma
[download]

"...dataset get significantly larger..."

Perhaps the solution with Iterator::Simple performs better. But I guess - and your mileage may vary. See also Benchmark.

Best regards, Karl