No write access to files in source archive

What command did you use to get the files?

itpartners@akarua:/tmp$ apt-get source libmail-spf-perl
Reading package lists... Done
Building dependency tree... Done
Need to get 73.7kB of source archives.
Get:1 http://archive.ubuntu.com feisty/universe mail-spf-perl 2.004-0ubuntu1 (dsc) [850B]
Get:2 http://archive.ubuntu.com feisty/universe mail-spf-perl 2.004-0ubuntu1 (tar) [69.5kB]
Get:3 http://archive.ubuntu.com feisty/universe mail-spf-perl 2.004-0ubuntu1 (diff) [3319B]
Fetched 73.7kB in 0s (140kB/s)
dpkg-source: extracting mail-spf-perl in mail-spf-perl-2.004
dpkg-source: unpacking mail-spf-perl_2.004.orig.tar.gz
dpkg-source: applying ./mail-spf-perl_2.004-0ubuntu1.diff.gz

itpartners@akarua:/tmp$ cd mail-spf-perl-2.004/

itpartners@akarua:/tmp/mail-spf-perl-2.004$ ls -la debian/changelog
-r--r--r-- 1 itpartners itpartners 487 2007-06-06 11:53 debian/changelog

It's not a bug in the packaging:

$ ls -la debian/changelog
-rw-r--r-- 1 kitterma kitterma 487 2007-06-05 20:51 debian/changelog

I suspect that the problem is that you made /tmp at / instead of in your home directory and so the permissions are off as a result. Try ~/tmp and see if it's different.

The problem was originally found in another directory; I just unpacked it in /tmp because I needed somewhere different.

itpartners@akarua:~/tmp$ apt-get source libmail-spf-perl
itpartners@akarua:~/tmp$ ls -la mail-spf-perl-2.004/debian/changelog
-r--r--r-- 1 itpartners itpartners 487 2007-06-06 13:25 mail-spf-perl-2.004/debian/changelog

This is the first package I've ever had this problem with.

I suspect the problem is that the files in the debian/ directory inside the orig.tar.gz have the incorrect permissions, so when the diff is applied, it also sets them 444.

And yet when I apt-get source the package, I don't have that problem. I believe it's something to do with permissions on your system and not with the packaging.