You need the if $] in the first definition of FOO. That should prevent it being constant-folded.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In reply to Re^9: Constant subroutine main::C redefined by tobyink
in thread Constant subroutine main::C redefined by BrowserUk

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