Although I'm not sure of the "why" at the moment (Update: see my other post), Overriding Built in Functions does say:

Overriding may be done only by importing the name from a module at compile time--ordinary predeclaration isn't good enough. However, the use subs pragma lets you, in effect, predeclare subs via the import syntax, and these names may then override built-in ones:
$ perl -wMstrict -le 'use subs "readpipe"; print `Bar`; sub readpipe {
+"Foo"}'
Foo

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In reply to Re: Overriding built-in function works only via import()? by haukex
in thread Overriding built-in function works only via import()? by perlancar

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