I happen to have access to an old freebsd machine that still runs 5.8.8 (hasn't been updated in ages...) and I see that the output looks like stuff that you've shown in earlier posts on this same general topic: 
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
tion which ensures
ion which ensures
on which ensures
n which ensures
which ensures that
hich ensures that
ich ensures that
ch ensures that
h ensures that
ensures that the
nsures that the
sures that the
ures that the
res that the
es that the
s that the
that the military
hat the military
at the military
t the military
the military always
he military always
e military always
military always obey
ilitary always obey
litary always obey
itary always obey
tary always obey
ary always obey
ry always obey
y always obey
always obey the
lways obey the
ways obey the
ays obey the
ys obey the
s obey the
obey the commands
bey the commands
ey the commands
y the commands
the commands of
he commands of
e commands of
commands of the
ommands of the
mmands of the
mands of the
ands of the
nds of the
ds of the
s of the
of the party.
f the party.

[download]

Well, because I first learned about the (?=...) regex construct (referred to as a "positive look-ahead assertion") when using 5.8.8, I have to assume there's something tricky and unexpected that happens when the look-ahead expression can match variable-length strings (and this was apparently fixed in 5.10).

If you really have to construct word trigrams in 5.8.8, I guess you'll have to do something other than a pure regex solution relying that sort of syntax. The easier way to do word n-grams is to use split: 

my $str1 = q/It is a guide to action which ensures that the military a
+lways obey the commands of the party./;
my $str2 = q/It is a guide to action that ensures that the military wi
+ll forever heed Party commands is a guide./;

for my $str ( $str1, $str2 ) {
    print "=== input is [$str] ===\n";
    my @words = split / /, $str;
    while ( @words >= 3 ) {
        print join( " ", @words[0..2] ), "\n";
        shift @words;
    }
    print "\n";
}

[download]
Of course, if that doesn't satisfy the contrived conditions of a given homework assignment ("You must not use 'split'! And you must use Perl 5.8.8!!"), well, that's a shame. The instructor ought to know better (or shouldn't be so deliberately mean to the poor students...)

In reply to Re: output differs in perl version by graff
in thread output differs in perl version by sarvan

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