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;
}