my %filehash;
{
    open my $fh, '<', '/path/to/file_one.txt' or die $!;
    $filehash{$_}++ while <$fh>;
    close $fh or die $!;
}

{
    open my $fh, '<', '/path/to/file_two.txt' or die $!;
    while (<$fh>) {
        print qq("$_" is not in file one\n)
            if not exists $filehash{$_};
    }
}