If I'm understanding you correctly, you're interested in finding all items in the first file that are not in the second file. If this is the case, List::Compare can assist you with this:
use File::Slurp qw/read_file/;
chomp( my @a_file = read_file 'a_file.txt' );
chomp( my @b_file = read_file 'b_file.txt' );
my @a_file_only = List::Compare->new( \@a_file, \@b_file )->get_Lonly;
print "$_\n" for @a_file_only;
Output on your data sets:
Hope this helps!