Personally, I want it to complain about any use of an undefined value.

That's so wrong. Paradoxically, making the use of undefined values always warn will have a net *drop* in useful warnings.

People use undefined values because they know warnings will occur if it's misused. If using undefined values always warns, a different value that never warns will be used instead.

It's true that having more warnings may turn people off and cause them to turn warnings off. Someone in my office did that right now, and I think it was for no warnings 'uninitialized'.

Nevertheless, I don't often use uninitialized values as actual values, and when I do, I check for it being !defined before using it further. So I want more restrictions on the use of uninitialized values. I would prefer if all arithmetic on undef values threw a warning, for example, or use in a boolean context.

Not to mention it's silly to have defined($undef), my $x = undef; and my $x = 123; warn.

Thank you for pointing out defined(): since it's defined to deal with undef, certainly it should not throw a warning with undef! Similarly with //, and exists is arguably in the same class.

However, "my $x = undef" is equivalent to "my $x" or even "undef my $x". And I don't consider "my $x = 123;" to use undef at all: in this context, you can't tell that $x was ever assigned undef, and even if it was, that value is never referenced but is instead stomped out of existence.

In C, the term I'm looking for is "rvalue": I would like warnings whenever I try to access the value of undef.

Thank you for the pointer to "no autovivification;" I don't know any details about autovivification and I need to look it up.

Nevertheless, I don't often use uninitialized values as actual values

You never optional fields in your data structures? You never use optional parameters? You never use output parameters (e.g. open)?

I use undefined values every 5 lines of code, I bet. Yes, you're telling me that one in five of my lines of code should warn.

And I don't consider "my $x = 123;" to use undef at all: in this context

That may be, but it is. You're doing sassign($x, 123) and $x is undefined, and you indicated you want all undefined operands to warn.

since it's defined to deal with undef, certainly it should not throw a warning with undef!

And of course, so are truth tests.

Thank god. I thought you wanted us to use

if (exists($h{x} && defined($h{x}) && $h{x})
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instead of

if ($h{x})
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I would like warnings whenever I try to access the value of undef.

Except you said you don't want those that expect undef as an argument to warn, so I don't know of a single thing you think should start warning that doesn't already.

Update: I just thought one. You surely want length($undef) to warn. That used to be the case, but that got fixed.

This way, the common check

if (defined($s) && length($s))
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can be written as

if (length($s))
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And it doesn't hurt since you still get a warning (instead of two) if you use length wrong.

my $length = length($undef);  # Used to warn here.
print $length;                # Still warn here.

my $length = length($undef);  # Used to warn here.
for (1..$length)              # Still warn here.

my $length = length($undef);  # Used to warn here.
pack 'N', $length             # Still warn here.

...
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As far as I can tell, the only change you want to have done is to remove a small but awesome improvement to Perl.