my %rhash = reverse %hash;
my $key = $rhash{$value};

##</code><code>##

my %rhash;
for (keys %hash) {
  no warnings 'uninitialized';
  push @{$rhash{$hash{$_}}}, $_;
}

print("$value keys: ", 
      join(', ', @{$rhash{$value}}),
      "\n");