Sure, you have a reference; you can modify it. I don't know if this is exactly what you want. It doesn't allow for keys which differ only by case having different values:

sub DeAOL {
    for ( keys %{$_[0]} ) {
        my $tmp = lc $_[0]->{$_};
        delete $_[0]->{$_};
        $_[0]->{lc $_} = $tmp;
    }
}
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After Compline,
Zaxo