use strict;
use warnings;

sub loop_over_hash {
    my( $keys, $href ) = @_;
    print join( ',', @$keys , $href ), "\n" and return unless 'HASH' eq ref $href;
    loop_over_hash( [ @$keys, $_ ], $href->{$_} ) for keys %$href;
}

my %hash = (
            a => {
                  1 => { x => 5, y => 6 },
                  2 => { s => 7, t => 8 }
                  },
            b => {
                  3 => 'end',
                 },
);

loop_over_hash [], \%hash;