In addition to what davido said, don't use curlies to initialize a hash. Your first line ( %a = {};) should be:

%hash = ();
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(%a doesn't really conflict with the $a that is used by the sort function, but why create a chance for confusion?)

Curlies are used like that when initializing a reference to a hash:

my $hashref = {};
$$hashref{level1_key}{level2_key}{level3_key} = "value";
...

for my $key1 ( keys %$hashref ) {
    for my $key2 ( keys %{$$hasref{$key1}} ) {
        for my $key3 ( keys %{$$hasref{$key1}{$key2}} ) {
            # do something with $$hashref{$key1}{$key2}{$key3}
        }
    }
}
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