Another solution using map. Does not pass an extra state variable around. More like a DFS approach. Handles array refs too. Uses brackets and curly brackets to differentiate between arrays and hashes.

sub flatten
{
    my ($value) = @_;

    my $rtype = ref $value or return "=$value\n"; # ends recursion

    if ( $rtype eq 'ARRAY' ) {
        return map {
            my $i = $_; map { "[$i]$_" } flatten( $value->[ $_ ] );
        } ( 0 .. $#$value );
    }
    elsif ( $rtype eq 'HASH' ) {
        return map {
            my $k = $_; map { "{$k}$_" } flatten( $value->{ $_ } );
        } sort keys %$value;
    }
    
    die 'flatten only handles scalars, arefs, or hrefs';
}

print for flatten( \%hohoh );
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