If they are package variables, you can use symbolic references:

my $sum = 0;
foreach my $i (1 .. 80) {
    no strict 'refs';
    $sum += ${"Sel${i}Ttl"};
}
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If not, you can use 'eval':

my $sum = 0;
foreach my $i (1 .. 80) {
    $sum += do {eval "$Sel${i}Ttl"}
}
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Or if you want to get fancy:

my $sum = 0;
local $" = "+";
eval "\$sum = @{[map {qq {\$Sel${_}Ttl}} 1 .. 80]}";
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All code untested.