Why does the first code work, while the second doesn't?

The Camel (🐪, Chapter 11) to the rescue:

Overriding Built-in Functions ... it's the assignment of a code reference to a typeglob that triggers the override, as in *open = \&myopen. Furthermore, the assignment must occur in some other package; this makes accidental overriding through typeglob aliasing intentionally difficult.

And looking at git history, the corresponding language was in perlsub as early as perl-5.000. So the behavior appears to be quite intentional.

And BTW, here's another short way to do what you want that doesn't use subs:

BEGIN { package Foo; *main::readpipe = sub {"Foo"} }
print `xyz`;
__END__
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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