> Is there a common idiom how to handle such case with signatures?

The rule of thumb is to use a default which is defined. °

Using undef as default should be redundant and will force you to check @_ - if still possible (untested)

Sorry, your code is longish (aka TLDR), if I didn't get it right, please consider condensing it to the relevant part.

This question could be actually very relevant if there is a real use case! P5P is considering to drop the population of @_ when signatures are used.

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

update

°) not 

# Standard signature with optional second arg.

sub sig_std ($what, $arg=undef) {

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In reply to Re: Distinguish between missing and undefined arguments with subroutine signatures by LanX
in thread Distinguish between missing and undefined arguments with subroutine signatures by jo37

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