merge-pod: Be more compatible with older perl

merge-pod: Be more compatible with older perl

merge-pod works fine with perl 5.00503, if we eliminate the 3-argument
invocation of open(). So, replace the open() calls with their
2-argument equivalent, and relax the version requirement a bit.

Change-Id: Ibeda39f2620ab1056e2d42838833d140ec3c053f
Reviewed-on: http://gerrit.openafs.org/3661
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>