According to the dictionary, "per" (or more conventionally "as per")
means "according to". Refer OED "per" sense II.3.a. For example:
"No value was passed, so return null, as per default".
In this sentence, we are not specifying the default, we are referring
to the default. This correct usage of "per default" was used nowhere
in MediaWiki core as far as I can see.
Instead we have "per default" being used to mean "by default", that is,
giving the value to use when no explicit value was specified.
In OED, the phrase "by default" is blessed with its own section just
for computing usage:
"P.1.e. Computing. As an option or setting adopted automatically by a
computer program whenever an alternative is not specified by the user
or programmer. Cf. sense I.7a."
There are highly similar pre-computing usages of the same phrase,
whereas the phrase "per default" is not mentioned.
As a matter of style, I think "per default" should not be used even
when it is strictly correct, since the common incorrect usage makes it
ambiguous and misleading.
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