open  fh, "<", "b.txt" or die $!;
open  fh1, "<", "a.txt" or die $!;

my $i = 0;
while(my $item1 = <fh>)
{
    open fh2, ">>", "a$i.txt" or die $!;
    while(my $item2 = <fh1>)
    {
        if($item1 =~ m/$item2/i)
        {
            print fh2 $item1, $/;
            print fh2 <fh>;
        }
    }
    close fh2;
    
    seek fh1, 0, 0;
}

close fh;
close fh1;