> So why isn't it documented?

Because contrary to $_ and @_ it has no function

> I could swear I saw it somewhere once upon a time, or maybe not...

People are exploiting the side effect that it's a global and doesn't need to be declared.

> What is it???

The typeglob / symbol _ is global to allow the scalar and array slots $_ and @_ to hold magic.

The hash slot is undefined but still global and can be (ab)used for syntactic sugar.

Like 

# No my/our %_ lives in  main::
%_ = (a => 1, b => 2);   # global variable in package main

[download]

See also this SO discussion https://stackoverflow.com/questions/36723739/undocumented-perl-variable#36724370

Edit

perldoc -v and perlvar should probably be expanded to show a generic explanation for such "reserved" special variables

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

In reply to Re^5: perldoc -lf anomaly (%_) by LanX
in thread perldoc -lf anomaly by Anonymous Monk

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