If a hole developed in a letter after writing (specifically in the interior of a heh or mem), is the tefillin still valid?

Synopsis

If a hole develops in the interior space of a heh or mem after writing, the Mechaber rules it valid. The Yerushalmi implies it needs surrounding parchment on the inside too. There is a dispute about holes in the left (inner) leg of a heh.

Early Acharonim
Lenient
Strict
Lesser cited
Major authority

Discussion

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