Well, the next major version of Perl comes with stricture enabled by default. (I don't know about warnings, though. But I guess they are enablabed by default as well).

The 5.x dev track has to maintain backwards compatibilty (for 99% or all cases, at least), so no way for default use strict;.

Update: In Perl 6 onliners via perl -e '...' are still non-strict ;-)