Now you realize, I mentioned only the "eq" and "ne" operators (cuz or/and/not already work on undef w/no warning).
I'm not talking concatenation, or where you print it out.
But specifically where you are testing the value against something else.
In that case, do you still think, that if you are doing a comparison on it, you wouldn't notice that "empty"(undef) is or is not equal to some other string?
How do you see it different than "and" "or" and "not"...

i.e.
string1 or string2 = true if one of them has length>0 etc...
but it doesn't warn you about undef.

It sounds like you were thinking concatenation... mixing strings w/strings... but if you are comparing, aren't you already testing the value? i.e.:

> perl -we 'use strict;use P; P ((undef and 1) ? "yes": "");'  
no
> perl -we 'use strict;use P; P ((undef or "") ? "yes": "no");' 
no
> perl -we 'use strict;use P; P ((undef xor "") ? "yes": "no");'
no
> perl -we 'use strict;use P; P ((undef eq "") ? "yes": "no");' 
Use of uninitialized value in string eq at -e line 1.
yes
 perl -we 'use strict;use P; P ((undef and 1) ? "yes": "no");'     
no
> perl -we 'use strict;use P; P ((undef and 1) ? "yes": "no");'
no
> perl -we 'use strict;use P; P ((2 or undef ) ? "yes": "no");'      y
+es
> perl -we 'use strict;use P; P ((2 xor undef ) ? "yes": "no");'
yes
perl -we 'use strict;use P; P ((2 ne undef ) ? "yes": "no");'   
Use of uninitialized value in string ne at -e line 1.
yes
Ishtar:/tmp> perl -we 'use strict;use P; P ((undef xor undef ) ? "yes"
+: "no");' 
no
 perl -we 'use strict;use P; P ((not undef xor not undef ) ? "yes": "n
+o");'
no
etc....
[download]

So you already have the ability to use defined/undefined as boolean states, why do you think a warning when using it with equality, (eq or ne), is useful?

Note I'm not talking about undef==3, or 2>=undef... or even undef cmp undef... as all of those *expect* some type of ordering or value.

But if something is eq or ne, doesn't.

Limiting it to that and considering we are only asking if what is in "$a" is the same as what's in "$b" (or not the same)...

You still think the warning for that specific case is more useful than not?