Further to hippo's post:   Note that going a step further and enabling strictures with  usestrict; would have prevented the code from compiling in the first place:

c:\@Work\Perl\monks>perl -le
"use strict;
 use warnings;
 ;;
 my @ar = ([1,12],[8,3],[4,57],[22,5]);
 my @sort = sort{$a[1]<=>$b[1]}@ar;
 foreach (@sort){
     print \"@$_\n\";}
"
Global symbol "@a" requires explicit package name at -e line 1.
Global symbol "@b" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
[download]

(I have to admit the  Global symbol "@a" requires explicit package name ... etc messages are slightly more obscure, but they explicitly point you to the offending variable.)

See also How do I sort an array by (anything)? in the Data: Arrays section of perlfaq4.

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