Close, but no cigar:

sort { $hash{$a} <=> $hash{$b} } values %hash;
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This is taking an array of hash values (returned by values %hash, and treating them as keys. And unless your values are also available as keys ( i.e. my %hash = ( key1 => 'key2', key2 => 'key1',); ) it will not give you the answer you are hoping for!

The best approach is to keep it simple:

# list of sorted keys in numeric order
my @keys = sort { $a <=> $b } keys %hash; 

# list of values in numeric order
my @values = sort { $a <=> $b } values %hash; 

# a list of the *keys* sorted by their corresponding value
my @keys_by_value = sort { $hash{$a} <=> $hash{$b} } keys %hash;
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Hope this makes things clearer!? Also remember their is plenty of other references out there on perldoc:

keys

values

sort

Good hunting!