Welcome to Perl and the Monastery, Eardrum!

Thank you for letting us know this is homework. We're happy to help, if you're willing to learn and show your efforts - the latter you've done by posting your code so far, thank you for that. Also take Corion's suggestions into account - for example, the other homework problems don't really have anything to do with this first question, you might consider removing them, or at least placing them inside <readmore>...</readmore> tags. See also How do I post a question effectively? and How do I change/delete my post?.

First, a couple of comments on your code:

• You're missing at least one semicolon, another line is missing an optional semicolon, it's a good habit to place them all - I hope you can see which two lines I'm talking about?

• It's very strongly recommended to Use strict and warnings, as they will help you catch potential problems.

• Although you've indented your code somewhat consistently, I'd suggest not indenting the for itself. Have a look at indentation styles on Wikipedia - pick one (such as the K&R style, personally I lean towards the "1TBS") and the most important thing is to consistently stick with that style. The tool perltidy can help with that. Normally you should be able to install it with the command cpanm Perl::Tidy (or cpan instead of cpanm, if you don't have App::cpanminus installed yet).

• In my ($limit)= <STDIN>;, you don't need the parens on the left hand side. They actually do something you might not expect: Instead of just reading one line from STDIN, the statement will read all lines from STDIN, and save only the first - if you enter a number and press return, your program will appear to not be doing anything, but it's really just waiting for more input (on a *NIX system, you can use Ctrl-D to end the input, as haj pointed out). If you use my $limit = <STDIN>;, it will read only one line.

  (What's going on here is that with parentheses on the left of an assignment, the right hand side will be evaluated in list context, which in the case of <filehandle> will try to read all lines, while without the parentheses, the right hand side is in scalar context, which reads one line. You can read more about this for example in the Context tutorial.)

As for your task, I found the problem statement "the average number from 1 to the number that the user inputs (in jumps of 2)" a little unclear, but the example (1+3+5)/3 = 3 helps. In this case, I think it helps to work out the steps "on paper":

1. The first step is to get all the numbers from 1 to 6 in steps of two. To see what your loop is doing, try putting a print statement at the top of the loop, such as print "i=$i\n"; - see also the tips on the Basic debugging checklist.

2. The next thing to do is sum up all of these numbers. You've got a variable $sum, which you're starting out at 1 and incrementing by 2 in the loop. I suggest you think about this as the main point: how could you use this variable to sum up the numbers from the previous step? Try adding another print statement or two to show how $sum changes.

3. Finally, you'll want to divide by the count of numbers that you summed up. In your code, you're pushing the string '1' into an @array and then using scalar(@array) to get the number of elements in the array after the loop. Although this works, consider what happens when $limit is a large number, such as in the millions or more: @array will contain a lot of elements. There's a more efficient way to count the number of times the loop runs - I'm sure you can come up with an idea :-)

A side note: you're using what's usually known as the C-style for loop. This is fine for cases like this one, where you need to count up by something other than one. For cases where you need to count up by ones, Perl offers another, nicer-looking mechanism which has the advantage of being less error prone (less chances for typos and off-by-one errors): the foreach loop (which can also be written for), and the Range Operators. The following prints all the numbers from 1 to 10:

for my $i (1..10) {
    print "i=$i\n";
}
[download]

(In Perl, There Is More Than One Way To Do It, so there are ways to count up by two's in this example too, but let's leave that for another time :-) )