use constant COMP => 0;
use constant USER => 1;
use constant UNKN => 2;

my %inventory;
open(my $fh_inv, '<', 'Inventory.csv') or die "Unable to open 'Inventory.csv' for reading: $!";
while ( <$fh_inv> ) {
    chomp;
    my ($key, $company, $user, $unkn) = (split /,/)[0, 3, 1, 4];
    $inventory{$key} = [$company, $user, $unkn];
}

my @Data;
open(my $fh_clients, '<', 'clients.txt') or die "Unable to open 'clients.txt' for reading: $!";
while ( <$fh_clients> ) {
    chomp;
    my @field = split /,/;
    my $iref = $inventory{$field[3]};
    if ( @$iref ) {
        push @Data, (join ',', $field[3], $iref->[COMP], @field[6,7,4,11], @{$iref}[USER, UNKN]);
    }
}
#processing @Data after this