my $current_key = 'Major task to perform?';
while ( my $choice = prompt( $current_key, ...)) {
    # set INDEX to offset of $choice in $structure{ $current_key }{answers}
    # if $structure{$current_key}{keys}[INDEX] is empty
    #        break out of loop; you;'re in a terminal state
    # else $current_key = $structure{$current_key}{keys}[INDEX]
}
<p>A lot of the code is left as an exercise to the reader. Good luck with your class.