@score is empty, because my(@names,@score) = list will put all the list elements in @names. If you want average scores try this instead:

open SCORES, "a:/scores.txt" or die "no scores!";
while(<SCORES>) {
    chomp;
    # turn the data into a hash, with names as keys
    # and scores as values
    my %scores = split(':',$input);

    # add up all the values
    my $total = 0;
    map { $total += $_; } values %scores;

    # divide the total by the number of names in the hash
    # to get the average.
    my $average = $total / (keys %scores);

    print "the average of the scores is $average\n";
}
close(SCORES);
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This assumes you only want an average of the scores on each line, and that the same name never appears twice on one line of your data file.