open(A_FILE, "<", "FILE_A.txt");

my(@a_lines) = <A_FILE>; # read file into list

close(A_FILE);

open(B_FILE, "<", "FILE_B.txt");

my(@b_lines) = <B_FILE>; # read file into list

my($b_lines);

close(B_FILE);

open(my $out, ">",  "Useful.txt") or die "Can't open Useful.txt: $!";

$number = @b_lines;

foreach $a_line (@a_lines) # loop thru list

{

    $found = 0;

    my $sentence = $a_line;

    $i = 0;

    chomp($sentence);

    while (($i <= $number-1) and ($found == 0)){

    chomp($b_lines[$i]);

    if ($sentence =~ /$b_lines[$i]/){

        $found = 1;

    }

    $i++;

    }

    if ($found == 1) {

        print $out $sentence."\n";

    }

}