What you are looking for is to store the rank of elements in @a in @b. (I think it was Randal who came up with the following idea first) -

use Data::Dumper;

$a[1]{total} = 10;
$a[2]{total} = 50;
$a[3]{total} = 40;
$a[4]{total} = 20;
$a[5]{total} = 30;

@b[sort{$a[$b]{total}<=>$a[$a]{total}}1..$#a] = 1..$#a;

print Dumper(\@b);
[download]

And the output is -

$VAR1 = [
          undef,   # this is ignored because index begins at 1
          5,
          1,
          2,
          4,
          3
        ];
[download]