Using variables of the same name in different scopes is fine as they are *new* variables within that scope (if they're lexical of course) e.g

my($foo,$bar) = qw(one two);
print "($foo, $bar)\n";
{
    # new scope, new variables
    my($foo,$bar) = qw(ichi ni);
    print "($foo, $bar)\n";
}
print "($foo, $bar)\n";

__output__

(one, two)
(ichi, ni)
(one, two)
[download]

But I'd highly recommend against using $a and $b as variable names as they are special to sort().
HTH

_________
broquaint