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
Prints:
75
-42
-28
6
-7
Use perldoc to look up chomp and defined if you aren't familiar with them.
True laziness is hard work
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