I strongly recommend you add use strict; to every script you write. Also you should always use the three parameter version of open and lexical file handles (a variable declared with my): open my $fileIn, '<', 'filename.txt' or die "Unable to open filename.txt: $!\n".

There are lots of ways to solve your problem and some of them don't need an array at all. Consider:

use strict;
use warnings;

my $lastValue = <DATA>;
chomp $lastValue;

while (<DATA>) {
    chomp;
    print $_ - $lastValue, "\n";
    $lastValue = $_;
}

__DATA__
1
76
34
6
12
5
[download]

Prints:

75
-42
-28
6
-7
[download]

Use perldoc to look up chomp and defined if you aren't familiar with them.