It doesn't sound to me like you want the indices of the array, but the actual values. When you use foreach (or for) with a scalar followed by an array, that's what you get. So you can just go ahead and use the values in your formula:

foreach my $value (@data) {
    $var = 1/($count-1);
    $variance = $var*(($value-$Average)**2);
}
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If you actually need the indices, you can use a C-style for loop or the range operator, in both cases letting the scalar value of the array represent the number of elements in it:

for( my $i=0; $i < @data; $i++ ){
  # do something with $data[$i]
}

# or 

for my $i (0..(@data-1)){
  # do something with $data[$i]
}
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