Reported in 5.61, seems to work okay in blead.
p4raw-id: //depot/perl@11921
EXPECT
Can't modify constant item in tie at - line 1, near ""Foo";"
Execution of - aborted due to compilation errors.
+######## undefing constant causes a segfault in 5.6.1 [ID 20010906.019]
+undef foo;
+EXPECT
+Can't modify constant item in undef operator at - line 1, near "foo;"
+Execution of - aborted due to compilation errors.