Warn when using a variable on the same statement it is locally masked

[caribpa]

Two days ago I spent quite some time debugging this code:

# a lot of code

my $num = 0;

# more code

sub test {
    # complex function

    return (my $num = shift) ? $num : 0;
}

# ...

say test 4;  # 0

# ...

When I found the bug I initially thought that the issue was related to the ternary operator. But after creating an issue (#17795) I learned that you cannot use variables on the same expression they are declared, excerpt shared by @Grinnz from the documentation:

The declared variable is not introduced (is not visible) until after the current statement.

Currently these will generate a compilation error under strict (Global symbol "$n" requires explicit package name) and a warning under warnings (Name "main::n" used only once: possible typo):

(my $n = shift) ? $n : 0; 

if ((my $n) || !!$n) { $n } else { 0 }

But this will only generate a compilation error under strict (nothing with warnings):

((my $n) || !!$n) ? $n : 0;  # Perl thinks it's fine when not using strict

Anyways, if you do something like the code I showed on the first example, Perl won't warn you nor generate a compilation error and will probably do what you did not mean to do:

my $num = 0;
{
    local @_ = 10;
    (my $num = shift) ? $num : 0;                         # 0
}
{
    local @_ = 10;
    if ((my $num = shift) && !!$num){ $num } else { 0 };  # 0
}

and I can say first hand that finding this bug is something really time-consuming with no pragma nor Deparse to help you.

I'd like to see a warning here saying that you are using a variable on the same statement you are lexically masking it.

Though I see that this warning would also appear on the likely more common case:

local @_ = @_;
local $_ = $_;
# and so