Everything is a matter of taste, and reading perl is a function of how you are used to seeing perl written, but I would go for more explicit var names and just a couple comments. The code itself is failry easy to read.

foreach my $line (@data) {
  my ($linecount, $filename) = split(/\s+/, $line);

  $filename =~ s/^$in_prefix//; # strip base directory from filename

  my ($campaign, $month, $fname) = split('/', $filename); # info is st
+ored in the directory nesting

  push @files, { campaign => $campaign,
                 month => $month,
                 file => $fname,
                 count => $count,
                 sort => "$campaign:$month:$file" };
}
[download]

It is a bit more verbose, and may be a bit slower because it uses explicit temporaries (instead of $_). I personally hate $_ and avoid it like the plague. I know this isn't a widely shared view.

But hey, you asked for opinions, right?

-jackdied