D:\>perl -wle"print undef"
Use of uninitialized value in print at -e line 1.

D:\>perl -wle"warn undef"
Use of uninitialized value in warn at -e line 1.
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D:\>perl -we"print 1 <=> undef
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
1
D:\>perl -we"print 1 + undef
Use of uninitialized value in addition (+) at -e line 1.
1
D:\>perl -we"print 1 - undef
Use of uninitialized value in subtraction (-) at -e line 1.
1
D:\>
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>> Pretty much everything else in Perl is happy to treat undef as 0, why not sort?
> I think you're confused
[...]

These are just warnings. sort dies, that's the difference. So if you don't have warnings on, you can do string comparisons with <=> as long as you want, and suddenly with a special input your script will die instead of only warn.
I see it as another encouragement to always use warnings =)

Whatcha talkin bout Willis?

D:\>perl -we"print for sort { warn qq,($a)($b),; $a <=> $b } 2, 1, 4 ,
+ undef, undef, 3, 'nan','nan','inf','inf'"
(2)(1) at -e line 1.
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
(4)() at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
Use of uninitialized value $a in concatenation (.) or string at -e lin
+e 1.
()(3) at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
(nan)(nan) at -e line 1.
(inf)(inf) at -e line 1.
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
(1)() at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
(1)(4) at -e line 1.
(2)(4) at -e line 1.
Use of uninitialized value $a in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
()() at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
Use of uninitialized value $a in concatenation (.) or string at -e lin
+e 1.
()(1) at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
(1)(3) at -e line 1.
(3)(2) at -e line 1.
(3)(4) at -e line 1.
(nan)(inf) at -e line 1.
(inf)(nan) at -e line 1.
Use of uninitialized value $a in concatenation (.) or string at -e lin
+e 1.
()(nan) at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
(nan)() at -e line 1.
Use of uninitialized value in numeric comparison (<=>) at -e line 1.
(nan)(1) at -e line 1.
(1)(nan) at -e line 1.
(1)(inf) at -e line 1.
(1)(inf) at -e line 1.
Use of uninitialized value $_ in print at -e line 1.
Use of uninitialized value $_ in print at -e line 1.
nannaninfinf1234
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I dont see how that can happen, except like

D:\>perl -e"print sort { $a <=> $b } undef, 1, undef, 2"
12
D:\>perl -e"print sort { $a <=> $b;undef } undef, 1, undef, 2"
Sort subroutine didn't return a numeric value at -e line 1.
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which is kind of stupid