The usual way would be to go through it recursively. Something like this:

#!/usr/bin/env perl
use Modern::Perl;

sub recurse {
    my $hashref = shift;
    for my $k (keys %$hashref){
        if( ref $hashref->{$k} eq 'HASH' ){
            recurse($hashref->{$k});
        } else {
            # do stuff with $key and $value
            say "$k -> $hashref->{$k}";
        }
    }
}

my %hash = ( one => { a => 11, b => 22, c => 33 },
             two => 222,
             three => { A => { aa => 1111 }, B => { bb => 2222 } },
             );
recurse(\%hash);
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I would also suspect that There Is A Module For That.