I was going to say that  use strict would have shown up your problem, but you're using $a and $b, so it probably wouldn't have been much use, after all. Bad $a. Bad $b.

edit:

[jasper@carabosse spamjab]$ perl -le 'use strict; my$x=99; my $x = $_ 
+for 1..3; print $x'

[jasper@carabosse spamjab]$ perl -le 'use strict; my$x=99; for (1..3){
+my $x=$_}; print $x'
99
[jasper@carabosse spamjab]$
[download]

curious? Is it?