use Modern::Perl;

my %keys;

while (<DATA>) {
    my ($key, $data1, $data2) = split /,/;
    unless (exists($keys{$key})){
        # do something
        say $key; # just as an example
        $keys{$key} = 1;
    }
}

__DATA__
key1, abd, 123
key2, dhde, 9+6+
key3, dxdc, edazedaz
key1, dea, 564dz
key1, deksizi, 4833
key4, eoz, 852662
key2, dzdadazd, 9566