It seems clear that Fletch has the right answer for you, but if you happen to be one of those people who likes to use existing shell utilities do your work for you, you could try it like this:

$destination = (split /\n/, `ls -dt /path/to/targets/PC*`)[0];
chdir $destination;
[download]

The options given to "ls" are: "d": list just the names of directories, not their contents; "t": order the listing newest-to-oldest. (You were using "r" with "t" which meant list the entries oldest-to-newest, and you weren't limiting the command to just entries starting with "PC".) Note that you don't need to include the "l" option (long, detailed format) to make "ls" order its output by date.