$ # override in BEGIN block works
$ perl -e'BEGIN { *CORE::GLOBAL::sin = sub { "a" } } print sin(0)'
a
$ # override at runtime doesn't have any effect
$ perl -e'*CORE::GLOBAL::sin = sub { "a" }; print sin(0)'
0
$ # using local, still no effect
$ perl -e'{ local *CORE::GLOBAL::sin = sub { "a" }; print sin(0) } print sin(0)
'
00