Not sure you will gain very much, but you could reduce these three regular expressions:

$line[4] =~ s/:/\//;
($month) = $line[4] =~ m/(\d+)\/\d+/;
($day) = $line[4] =~ m/\d+\/(\d+)/;
[download]

to only one:

my (month, $day) = $line[4]  =~ m/(\d+):(\d+)/;
[download]

But the thing that you should really do to start with is to profile your code, using for example http://search.cpan.org/~timb/Devel-NYTProf-5.06/lib/Devel/NYTProf.pm. Then only you will know where your program is spending time.