Here is an example of how you could recurse through a structure of hash references in practice.

It uses the ref function to determine whether a given level is a hash reference.

use Modern::Perl;

my %ref_chain = ( 
    'key1' => {
        'key2' => {
            'key3' => 'not a reference'    
        }    
    },
    'key4' => {
        'key5' => 'no ref here'    
    }    
);

sub traverse_hashes {
    my $hashref = shift;
    foreach my $key (keys %$hashref)
    {
        if (ref $hashref->{$key} eq 'HASH')
        {
            say "$key is a hash ref. Recursing.";
            traverse_hashes($hashref->{$key});    
        }
        else
        {
            say "$key is not a hash ref (value: $hashref->{$key})";
        }
    }    
}

traverse_hashes(\%ref_chain);
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