use strict;
use warnings;

my $first_file  = shift || 'a.txt';
my $second_file = shift || 'b.txt';

open my $a_fh, '<', $first_file  or die "$first_file: $!";
open my $b_fh, '<', $second_file or die "$second_file: $!";

my %second_file;
@second_file{map { unpack 'A*', $_ } <$b_fh>} = ();

while (<$a_fh>) {
    print unless exists $second_file{unpack 'A*', $_};
}