I don't know the modern modules that well, but the for do-it-yourself approach, I guess if you want arbitrary depth then a hash walker might look like:

use warnings;
use strict;

my $hoh = {
    '1' => {
        '1a' => {}
    },
    '2' => {
        '2a' => {
            '2b' => {}
        },
        '2c' => { X => 1 },
    }
};
# This coderef examines every node to see if it is the
# one we want
my $evaluator = sub {
    my $item = shift;
    return (ref $item
            && ref $item eq 'HASH'
            && exists $item->{X});
};

my $path = walk_hoh($hoh, $evaluator);
print "found: ", join(", ", @$path), "\n";

exit 0;

sub walk_hoh
{
    my $hr = shift;
    my $evaluator = shift;
    my $keypath = shift || [];

    foreach my $key (keys %$hr) {
      my $val = $hr->{$key};
      if ($evaluator->($val)) {
          # We found it here
          return [@$keypath, $key];
      }
      if (ref $val && ref $val eq 'HASH') {
        # To iterate is human, to recurse divine   
        my $found = walk_hoh($val,
                             $evaluator,
                             [@$keypath, $key]);
        return $found if (defined $found)         
      }
    }
    return undef; # Found nothing here
}
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