The following (improper?) code segfaults:
.namespace ['Foo']
.sub __invoke :method
.param pmc a
say 'hi'
.end
.sub main :main
$P0 = newclass "Foo"
$P1 = new "Foo"
$P1()
.end
Removing the __invoke method generates a method not found error, avoiding
the segfault. Removing the .param also avoids the segfault. Changing the
override to "invoke :vtable" avoids the segault.