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D:\>perl -we"print for sort { warn qq,($a)($b),; $a <=> $b } 2, 1, 4 ,
+ undef, undef, 3, 'nan','nan','inf','inf'"
...
nannaninfinf1234

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.