According to my tests, perl 5.12.3 gives the following incorrect output (same as in your post):

It is a
is a guide
a guide to
guide to action
to action which
action which ensures
which ensures that
ensures that the
that the military
the military always
military always obey
always obey the
obey the commands
the commands of
commands of the
of the party.
[download]

However, perl 5.14.0 gives the correct output:

It is a
t is a
is a guide
s a guide
a guide to
guide to action
uide to action
ide to action
de to action
e to action
to action which
o action which
action which ensures
ction which ensures
...
[download]

Does someone have a perl 5.12.4 handy to test whether this bug is still present in the 5.12 branch?