Just iterate over the keys on each level of the hash: 
my %h2;
for my $level1 (keys %HASH) {
    for my $level2 (keys %{ $HASH{$level1} }) {
        $h2{$level2}{$level1} = $HASH{$level1}{$level2};
    }
}
%HASH = %h2;

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: inverting keys in a 2-D Hash by choroba
in thread inverting keys in a 2-D Hash by melmoth

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