If you wanted to do it using map you could do this: 
my %hash;

map { push( @{ $hash{ $values{$_} } }, $_ ) } keys %values;

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which if you were golfing would save you a few characters on Roy Johnson's probably easier to read version: 
while(my($k,$v)=each%values){push@{$hash{$v}},$k;}

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which becomes: 
map{push(@{$hash{$values{$_}}},$_)}keys%values;

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and then access the result with: 
for my $key (reverse sort keys %hash) {

    print "Level $key:\n", join(", ", @{$hash{$key}}), "\n\n";

}

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In reply to Re: Sorting hashes with new value detection by serf
in thread Sorting hashes with new value detection by sulfericacid

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