Well, since i don't know the answer to your pos question, allow me to do another side step and solve the original while loop/regex combo.

The problem is that in order for the while loop to finish, there has to be nothing left to match. Try this instead:

my $a = '12345';
while($a =~ s/(.)(.)//) {
    my ($b,$c) = ($1,$2);
    print "$b - $c\n";
}
[download]

But notice that '5' gets skipped. Better than getting caught in an infinite loop i suppose. I still recommend testing for an even lengthed string first. Another consequence is $a is completely destroyed if it is an even number of chars long, and contains the last char if it was odd lengthed.

UPDATE:
Your CB comment is absolutely correct - i was wrong in assuming that you have to have s/// to solve this one:

while($a =~ /(.)(.)/g){
    my ($b,$c) = ($1,$2);
    print "$b - $c\n";
}
[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)