$ perl -wMstrict -Mdiagnostics -e 'my $x if 0'
Deprecated use of my() in false conditional. This will be a fatal error in Perl
	5.30 at -e line 1 (#1)
    (D deprecated) You used a declaration similar to my $x if 0.  There
    has been a long-standing bug in Perl that causes a lexical variable
    not to be cleared at scope exit when its declaration includes a false
    conditional.  Some people have exploited this bug to achieve a kind of
    static variable.  Since we intend to fix this bug, we don't want people
    relying on this behavior.  You can achieve a similar static effect by
    declaring the variable in a separate block outside the function, eg
    
        sub f { my $x if 0; return $x++ }
    
    becomes
    
        { my $x; sub f { return $x++ } }
    
    Beginning with perl 5.10.0, you can also use state variables to have
    lexicals that are initialized only once (see feature):
    
        sub f { state $x; return $x++ }
    
    This use of my() in a false conditional has been deprecated since
    Perl 5.10, and it will become a fatal error in Perl 5.30.