'abc'
'def'
'121'
'xyx'
 

##</code><code>##

'def'
'121'
 

##</code><code>##

'121'
'abc'

'121'

'xyx'
 

##</code><code>##

use strict;
use warnings;
my $f2 = 'cat_mapping_in_A.txt';
my $f1 = 'cat_mapping_in_B.txt';
my $outfile = '1.txt';
my %results = ();
open FILE1, "$f1" or die "Could not open file: $! \n";
while(my $line = <FILE1>){
   $results{$line}=1;
}
close(FILE1);
open FILE2, "$f2" or die "Could not open file: $! \n";
while(my $line =<FILE2>) {
   $results{$line}++;
}
close(FILE2);
open (OUTFILE, ">$outfile") or die "Cannot open $outfile for writing \n";
foreach my $line (keys %results) {
   print OUTFILE "$line\n" if $results{$line} == 1;
}
close OUTFILE;