Which version are you using?

Looks like you should upgrade ...

$ perl -v

This is perl, v5.8.5 built for i386-linux-thread-multi

Copyright 1987-2004, Larry Wall
[SNIP]

$ perl -le '$_ = ($x) = (4, 5, 6); print'
3

$ perl -MO=Deparse,-p -le '$_ = ($x) = (4, 5, 6); print'
BEGIN { $/ = "\n"; $\ = "\n"; }
($_ = (($x) = (4, 5, 6)));
print($_);
-e syntax OK

$ perl -MO=Deparse,-p -le '$_ = ($x) = (4, 5, 6); print' | perl
-e syntax OK
3
[download]

See also Parsing of subscript separator in B::Deparse for another Deparse case.

HTH