my @patterns1 = ('foo') x 2;
my @patterns2 = ('bar') x 2;

use re 'debug';
while (@patterns1) {
    'a' =~ shift @patterns1;
    'b' =~ shift @patterns2;
}

__END__

Compiling REx `foo'
size 3 first at 1
   1: EXACT <foo>(3)
   3: END(0)
anchored `foo' at 0 (checking anchored isall) minlen 3
Compiling REx `bar'
size 3 first at 1
   1: EXACT <bar>(3)
   3: END(0)
anchored `bar' at 0 (checking anchored isall) minlen 3
Freeing REx: `foo'
Freeing REx: `bar'