Hi, I don't have a step by step guide, but Perl is installed by default on Mac OS X. The default Perl install is in /usr/bin/perl

bruce:0:~/tmp $ type perl
perl is /usr/bin/perl
bruce:0:~/tmp $ ls -l /usr/bin/perl
-rwxr-xr-x   1 root  wheel  43444 Nov 16  2006 /usr/bin/perl
bruce:0:~/tmp $ perl -v

This is perl, v5.8.6 built for darwin-thread-multi-2level
(with 3 registered patches, see perl -V for more detail)
[download]

It sounds like you have to change your PATH so that the ActiveState Perl is found before the default installation of Perl.

I can't tell you where ActiveState Perl is installed but I would start with /usr/local/ActiveState/bin/perl judging by the location of the DateTime module.

You could use the find command, i.e.  find /usr -name perl