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 line 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 line 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 line 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 line 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



##</code><code>##

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.