%nodes=( root => { parents => [],
                   probs => [0.01]}, # P(root)
         some => { parents => ['n1'],
                   probs => [0.74,0.21]}, # P(some|!root), P(some|root)
         other =>{ parents => ['root','n1'],
                   probs => [0.12,0.24,   # P(other|!root,!some), P(other|!root,some)
                             0.81,0.18]}, # P(other|root,!some), P(other|root,some)
         );

##</code><code>##

# the state you start from
%state=(root=>1,some=>0);
# let's get P(other|root,!some)
$i=0;
for @{$nodes{other}{parents}} {
 $i=($i<<2)+$state{$_} # this is a simple encoding. Just use the same for updating and accessing
}
$prob=$nodes{other}{probs}[$i];

##</code><code>##

use XML::Simple;
print XMLout(\%nodes);