I'd go for a HoH, although a HoA would work. Consider:

use strict;
use warnings;
use Text::CSV;

my %lookup;
my $csv = Text::CSV->new ();
my $filename = 'DATA';

while (<DATA>) {
    next unless $csv->parse ($_);

    my @row = $csv->fields ();
    next unless @row >= 7;
    
    my $key = $row[0];
    next if exists $lookup{$key} && $row[7] <= $lookup{$key}{max};
    
    $lookup{$key} = {max => $row[7], line => $., file => $filename};
}


print "Following maximums found:\n";

print "$_ = $lookup{$_}{max} in file $lookup{$_}{file}, line $lookup{$
+_}{line}\n"
    for sort keys %lookup;
    

__DATA__
DOW,Sep,18,09:31:16,440,29,142,10148,4
Russell2000,Sep,18,09:31:16,440,29,142,10148,4
RussellComposite,Sep,18,09:31:16,440,29,142,10148,4
Russell1000,Sep,18,09:31:16,440,29,142,10148,4
SP500,Sep,18,09:31:16,440,29,142,10148,4
Russell2000,Sep,18,09:31:16,440,29,142,10120,4
SP500,Sep,18,09:31:16,440,29,142,10160,4
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Prints:

Following maximums found:
DOW = 10148 in file DATA, line 1
Russell1000 = 10148 in file DATA, line 4
Russell2000 = 10148 in file DATA, line 2
RussellComposite = 10148 in file DATA, line 3
SP500 = 10160 in file DATA, line 7
[download]