print($foo, exit);  # Obviously not what you want.
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The print function is called with two arguments, $foo, and exit. Both will be evaluated to get at their values, and these values will then be sent to the print function to print out. That is, $foo evaluates to whatever is stored in it (say, the string "hi"), and then exit is evaluated by calling the exit function, which aborts the program. So print never gets a chance to print anything.

This is rather basic. More interesting is why this prints something:

print ($foo), exit;
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Here the expression consists of a call to the print function with one argument, and then a comma, and then a call to the exit function. As a comma operator evaluates its left argument, throws the return value away, and the evaluates its right argument, first the call to print($foo) is evaluated, resulting in a call to print, this in turn will evaluate $foo and send its value to the print function (making it print out something like "hi"), and then second, the call to exit is evaluated, aborting the program.

You should read Structure and Interpretation of Computer Programs.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com