I am running This is perl, v5.8.5 built for i686-linux-thread-multi
The problem you saw can be reproduced here too; here's an interesting thing though:

perl -MO=Deparse,-p -e '$_ = ($x) = (4, 5, 6); print'
($_ = (($x) = (4, 5, 6)));
print($_);
-e syntax OK
[download]

And here's the output of what Deparse gave me:

perl -e '($_ = (($x) = (4, 5, 6)));print'
3#and my shell prompt here of course
[download]

Note: that I removed the -l switch