use Tie::File;
tie my @file_array, 'Tie::File', $filename

foreach (@file_array) {
    if (/^([^:]+)(?::[^:]*){6}:([^:+]+):/) {     # this regexp can probably be improved.
        $users{$1} = $2;
        # you can add a debug print here if you want.
    }
}
untie @file_array;