I didn't know this one until I saw ccp's answer and I don't know of a second way.
*foo = *`; $* = 1; "true\n0"=~/\z/;
*| = \$foo;
2 =~ /(2)/; *| = *^N;
xmath gave this one to me because it contradicted the documentation and it isn't supposed to be possible. There is a loophole but it requires reading an obscure bit from Changes5.8.