open my $fhA, '<', '1.dat' or die $!;
open my $fhB, '<', '2.dat' or die $!;
while (my $lineA=<$fhA>){
    seek $fhB, 0, 0;
    while(my $lineB=<$fhB>){
        print $lineA,' ', $lineB;
    }
}

##</code><code>##

open $fhA, '<', '1.dat' or die $!;
open $fhB, '<', '2.dat' or die $!;
@B_lines = <$fhB>;
while (my $lineA=<$fhA>){
    for my $lineB (@B_lines) {
        print $lineA,' ', $lineB;
    }
}