The basic problem is that your regex is capturing the leading pipe, so that to <=> you end up passing '|10' and '|5'. Not surprisingly, these values are both numerically the same as 0.

Of course, the extra leading pipe character doesn't affect the result of a cmp operation.

Changing your code as little as possible, this will do what you want - though I must say that the entire code design still makes me recoil, and I'd jump to a transform-sort-untransform structure as the other commentor is pointing out:

@lines = sort {($b =~ /(?:\|([\w\s]+)){2}/)[0] <=> ($a =~ /(?:\|([\w\s
+]+)){2}/)[0]
     } @lines;
[download]

By the way, I discovered what was going on with judicious use of Data::Dumper:

$b = "0|aa aa|1998|aaa a|a aaa|10|aa a aa";
@a = ($b =~ /(\|[\w\s]+){2}/);
use Data::Dumper;
print Dumper(\@a);
[download]

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/
[download]