Pretty much everything else in Perl is happy to treat undef as 0, why not sort?
I think you're confused 
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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Nothing in perl treats undef as 0 without barfing 
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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In reply to Re^3: Dangerous Names by Anonymous Monk
in thread Dangerous Names by samtregar

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