interesting idea ! I've never tried this. I would probably go for a slightly different design to get round the problem. something like an array of array or even hash of arrays

my(@sum);
$var=1;
$sum[$var]=[1,2,3];
$var=4;
$sum[$var]=[5,7,8];

or ( better );

my(%sum)
$var=1
$sum{$var}=[1,2,3]
$var=5
$sum{$var}=[5,6,7]
[download]

hope it helps wertert