Hello and welcome!

Please see How do I compute the difference of two arrays in perlfaq4. You can also find this information on the command line via

perldoc -q difference
[download]

Also, maybe for a fancier difference, you might want Algorithm::Diff.

Welcome

Check out How do I post a question effectively?. It will suggest that you enclose your code in tags:

<code>
...your program...
</code>

This will make it readable like so:

use strict;
use warnings;

my $f1 = "C:\\Documents and Settings\\KXDC7647\\Desktop\\file1.txt";
my $f2 = "C:\\Documents and Settings\\KXDC7647\\Desktop\\file2.txt";
my %results;

# CONSTRUCTING HASH ARRAY BY CONTENTS OF FILE1
# IT WILL IGNORE LINE IF IT CONTAINS WHITE SPACE AT BEGINNING
open FILE1, "$f1" or die "Could not open file: $! \n";
while (my $line = <FILE1>) {
    if ($line =~ m/^\s*$/) {
        next;
    }
    else {
        $results{$line} = 1;
    }
}

# CONSTRUCTING HASH ARRAY BY CONTENTS OF FILE2
open FILE2, "$f2" or die "Could not open file: $! \n";
while (my $line = <FILE2>) {
    if ($line =~ m/^\s*$/) {
        next;
    }
    else {
        $results{$line}++;
    }
}
close(FILE1);
close(FILE2);

# PRINTING THE DIFFERENCE IN PERL
foreach my $content (keys %results) {
    print $content if values(%results) == 1;
}
foreach my $line (keys %results) {
    print $line if $results{$line} == 1;
}
[download]

And welcome to the forum! Hang around, and you'll learn our ways. You'll find it an enriching experience.