I'm totally guessing, because I don't have sample data, but I think you want to do something like this:

  Open both files
  Read each line from the first file
  Split it into mount point, total space, used space, free space
  Do the same for the second file
  Compare matching mount points to see the differences.
[download]

So here's a skeleton program. Notice that the difference between your program and mine is that I'm recording the data in a pair of hashes so I can cross-check them later. I also switched the open() to use the three-arg form to make it clear you mean to read these files. The code to read them is identical, so I pushed it down into a subroutine that creates the hash from the file and then gives it back to the caller.

open( my $firstfile,  '<', "DFLOG1.txt"); 
open( my $secondfile, '<', "DFLOG2.txt"); 

my %first_machine  = consume($firstfile);
my %second_machine = consume($secondfile);

foreach my $mount_point (keys %first_machine) {
    if ( exists $second_machine{$mount_point} ) {
        # Perform calculations here.
        # $first_machine{$mount_point}->[0] is the total
        # $first_machine{$mount_point}->[1] is the used
        # $first_machine{$mount_point}->[2] is the free
        # Similar for $second_machine.
        # Print here after doing calculation.
    }
    else {
        print "No mount point corresponding to $mount_point on the sec
+ond machine.\n";
    }
}

sub consume {
    my ($filehandle) = @_;
    my %result;
    while ( defined($_ = <$filehandle>) ) {
        chomp; 
        next unless /dev/;
        ($mount_point, $total_space, $used_space, $free_space) = split
+;
         $result{$mount_point} = [$total_space, $used_space, $free_spa
+ce];
    }
    return %result;
}
[download]

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