/\.\d+/
[download]

And based on the patterns (examples) he gave before, I don't think we can assume there's a dot, but we do know the numbers are at the end. Therefore:

/\d+$/
[download]

Furthermore, the whole thing can be reduced to this:

my @directories = grep { /\d+$/ and -d $_ } readdir DIR;
[download]

although that doesn't do the print. But I don't think that's necessary.