brackysclereid:

my $first_item = $List[0];
my $fifth_item = $List[4];
print $fifth_item-$first_item, "\n";
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Update:

for my $idx = (1 .. $#List) {
   my $val = $List[$idx] - $List[$idx-1];
   print "List[n] - List[n-1] = ", $val, " for n=", $idx, "\n";
}
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...roboticus

When your only tool is a hammer, all problems look like your thumb.

Have a look at the section on array variables in perlintro and it will explain how to get the value of an array element, and lots of other things too ;)

use strict;

my @List;

open(FILE, "filename.txt") or die "Unable to open file\n";

# assuming numbers, get them from file and add them to your array

while(<FILE>){
   #each time through the loop, line is held in $_
   chomp $_; #remove any newline from the line taken from file
   push(@List,$_); #push line into the array, in your case the number 
+you got
}   

close(FILE);

# to loop through the array, I found it usefull to increment my count 
+var ($i) by two
# here, scalar(@List), returns the number of elements in the array.

for(my $i=0;$i < scalar(@List);$i+=2){

   my $answer = ($List[$i+1]-$List[$i]); #use array indexing for your 
+calculation
   print "$answer\n";

}
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Thanks for all the excellent information! I have learned more about array manipulation in the last hour than in the last day! I really appreciate the well annotated code!

#!/usr/bin/perl
print "newbie thanks you all! \n"
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Cheers!, Art

# to loop through the array, I found it usefull to increment my count 
+var ($i) by two
# here, scalar(@List), returns the number of elements in the array.

for(my $i=0;$i < scalar(@List);$i+=2){
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Why increment by two?

Lotus1: can't remember why I did, hah, but maybe I misunderstood what he was trying to do.

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
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Prints:

75
-42
-28
6
-7
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Use perldoc to look up chomp and defined if you aren't familiar with them.