sub find_path {
    my ($beg, $end, $net, $seen, $stop, $work) = @_;
    if (! defined $stop) {
        %$stop = map {$_ => 1} @{$net->{$end}};
        for (@{$net->{$beg}}) {
            return "$beg=>$_" if $_ eq $end; 
            push @$work, {key => $_, path => "$beg=>$_"};
        }
    }
    my $next = [];
    for (@$work) {
        my ($node, $path) = @{$_}{qw/key path/};
        next if $seen->{$node}++;
        return $path if $node eq $end;
        return "$path=>$end" if $stop->{$node};
        push @$next, map {key => $_, path => "$path=>$_"}, @{$net->{$node}};
    }
    return find_path($beg, $end, $net, $seen, $stop, $next) if @$next;
    return 'path not found';
}