when the perl5-porters update the documentation, and say "don't do that its undefined", then they mean it don't do that even if does something you expect 80% of the time :)

For example see Re^3: question 1st - undefined behaviour (perlop)

I'm sure there is even more similar nodes/posts of wisdom to be found for Perl::Critic::Policy::Variables::ProhibitConditionalDeclarations ... trust the documentation

update:
my $cache = undef if undef;
Re^4: Managing capture
"my $p = value if condition()" as bad|deprecated as "my $p if 0" (Naughty indeed)
This Week on p5p 2000/05/21 my $x if 0; Trick
my $x if 0; Trick my @array = () if 0;
state variables, first draft - nntp.perl.org
state