I'll explain the map section for you.

my @files = grep { !/.+$/ && -M $_ > 1; } map { $mydir."/".$_; } readdir(DIR);

is basicly the same as

# get the base file names
my @files = readdir(DIR);
# for each $file in @files, append the path to the front
@files = map { $mydir."/".$_; } @files;
# for each $file in @files, if it doesn't match the criteria then remo
+ve it from @files
@files = grep { !/.+$/ && -M $_ > 1; } @files;
[download]

I hope that I didn't confuse you even more (:

my @files = grep { !/^\.+$/ && -M $_ > 1; }
            map { $mydir."/".$_; } readdir(DIR);
[download]

There is no point in doing that regex (corrected to your version above) after map, it will always fail. Also, you may want to skip all dot-files, since they are supposed to be hidden.

my @files = grep { not /^\./ and -M "$mydir/$_" > 1 } readdir(DIR);

— Arien