Unless you have disk issues, you can safely let the two co-exist (at least on Linux and Solaris - I don't have access to other platforms). You should still have an executable named perl5.10.1 in the install directory's bin subdir.

Under the lib subdir, you will have subdirs corresponding to the version (5.10.?, 5.12.1) that will contain the modules that are appropriate to the version, so you should still be able to run.

For instance, since my development perl is under /usr/local/perl, and I have 5.8.9, 5.10.1 and 5.12.1 installed, I have /usr/local/perl/bin/perl, which is hard linked to /usr/local/bin/perl5.12.1. I also have perl5.8.9 and perl5.10.1 in the same directory.

Under /usr/local/perl/lib, there are 5.8.9, 5.10.1 and 5.12.1 subdirs (also under /usr/local/perl/lib/site_perl). They will all operate correctly.

If you simply *must* remove the 5.12.1 capability, you *may* be able to use apt to downgrade your perl install.

If disk space is not an issue (and you must use 5.10.1), execute the following:

cd $(dirname $(which perl))
[download]

This will change to the directory where the perl binary is located.
Now, find the perl 5.10.1 binary and compare it to the perl binary (I am showing mine, which is 5.12.1:

ls -i perl && ls -i perl5.10.1 && ls -i perl5.12.1
1198045 perl
1156173 perl5.10.1
1198045 perl5.12.1
[download]

The first column is the starting inode number for each file. Note that on mine, both perl and perl5.12.1 point at the same file, while perl5.10.1 is a different file. If this is the case on your machine, you can do the following:

sudo rm -f perl
sudo ln perl5.10.1 perl
[download]

Now, any perl module you run that starts with #!/usr/bin/perl (or wherever your perl executable is located) will use v5.10.1, while you will still have 5.12.1 available.

Good luck.