What are the use cases of that pattern?

The pattern you are observing is that the package directive controls the package in which code is compiled. sub X::foo { pp(\@_) } is simply not an exception to that. Effort to provide a special behaviour for sub X::foo { pp(\@_) } was not spent.

Put differently, 

sub X::foo { pp(\@_) }

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is short for 
BEGIN { *X::foo = sub { pp(\@_) }; }

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which is effectively what happens every time you import a symbol from a module (e.g. use X qw( foo );).

In reply to Re: Use cases for 'sub Pckg::func { }' ? by ikegami
in thread Use cases for 'sub Pckg::func { }' ? by LanX

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