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.

##</code><code>##

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 second 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_space];
    }
    return %result;
}