I can only guess at what you intended but here's what you probably meant to say. (in two different versions). You'll probably also want to use a different variable name than $a because that is special and will be clobbered if you use sort(). ($b is also special, skip that one too).

# Better
for (@score) {
    print "$_\n";
    $a += $_;
}
$dev += $#score;
print $dev;

# Less nice
for (my $ctr = 0; $ctr < @score; $ctr++ ) {
    print "$score[$ctr]\n";
    $a += $score[$ctr];
}
$dev += $#score;
print $dev;
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