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";
}
[download]

...roboticus

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