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%reverseHash = reverse %hash;
%hash = reverse %reverseHash;

my %reverseHash;
while(($key, $value)=each %hash)
{
...

my %newHash;
@newHash{@keys} = @hash{@keys};

use Data::Dumper;
my %hash = (
  a => [1, 2],
...
my %reverseHash = map { Dumper($hash{$_}) => $_ } keys %hash;
%hash = map { $_ => $hash{$_} } values %reverseHash;
print Dumper \%hash;