This will only work is the values are unique but all you need to do is create a reversed hash lookup table and use that:

my %h = qw( 1 2 3 4 5 6 );
my %rev_h = reverse %h;
use Data::Dumper;
print Data::Dumper::Dumper(\%h,\%rev_h);
__DATA__
$VAR1 = {
          '1' => '2',
          '3' => '4',
          '5' => '6'
        };
$VAR2 = {
          '6' => '5',
          '4' => '3',
          '2' => '1'
        };
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If you can't guarantee the values are unique then you have to iterate. Use grep:

my @keys = grep{ $hash{$_} eq $find } keys %hash;
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