The first expression is run before entering the loop. Before each iteration the second expression is run; if false, the loop terminates. After each iteration, the third expression is run. For further details, see man perlsyn.

Note that there are semi-colons between the expressions, not commas.

In this case, the OP has constructed a foreach loop iterating $_ over a list of three elements: $i=500, $i=900 , $i += 100, which evaluates to 500, 900, 1000 and $i set to 1000 for ALL iterations of the loop.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

Ugh the carelessness.. Thank you, I just needed to realize semi colons were to be used.

Perls three-argument for loop is exactly like in C: initializer, test, and counting. See also http://en.wikipedia.org/wiki/For_loop#Three-expression_for_loops. Other languages may implement for loops differently, notably BASIC and its derivates often have constructs like for i=1 to 100 step 3 (counting up from 1 with increments to 3 until 100 is reached or exceeded) or even for i=100 downto 1 step 3 (counting down in steps of 3, starting at 100, stopping when 1 is reached or exceeded). Other languages have even stranger for loops, see the wikipedia article.

Alexander

I was taught that for structure was like this: (starting value, ending value, increment). But I infer from your response that it must be: (starting value, continue while this is true, increment)?

Won't hurt you to read the docs...

What's wrong:

• The first expression has nothing to do with a "starting value". (Starting value of what??? It's not like C-style loops have counters.)

• The last expression has nothing to do with an "increment". (Incrementing what??? It's not like C-style loops have counters.)

What it really means

• The first expression (if specified) is evaluated unconditionally before the loop is started.

• The second expression is evaluated at the start of every loop pass (incl the first). If an expression is specified and it evaluates to something false, the look exits.

• The third expression (if specified) is evaluated at the end of every loop pass (even if next is called).

A common usage:

for (my $i = 0; $i < $n; ++$i) {
   ...
}
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The above is equivalent to the following, except $i is scoped to the loop.

my $i = 0;
while ($i < $n) {
   ...
} continue {
   ++$i
}
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