I'm trying to edit the script so it can work out three arguments. Please can someone help.

You neglected to mention how it does not work for you. So, here is my guess. If you change:

exit(1) unless $nbArguments == 5;
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to:

exit(1) unless $nbArguments == 4;
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Then execute your code like this:

C:\Perl> 837421.pl 1 + 3 4
number of arguments: 1 
+ 3 
+ 4 
= 8
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It seems to add the 3 numbers. The key is to count the number of arguments properly.

Here are a few more tips:

• Use perltidy to neaten up the code formatting so that it is easier for you (and especially others) to understand.
• use strict and warnings so that you can catch typos like $nbArgumentsn.
• Don't name your variables $a and $b because they have special meaning (see sort).
• Add some usage documentation (preferably using POD) so that you remember how to run your code 6 months from now.