The nub of the problem is that most people expect that in the following code, 
my $x = 1;
sub foo { $x }

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There is a single variable called $x which is accessible from the main program and from within foo. The way it happens to be implemented internally is that they are treated as two separate variables that happen to be initially aliased to the same value. Here's a little ASCII diagram.

Before the for loop; the two X's point to the same value 

$MAIN_X  ---
            \
             -> +---+
$FOO_X  ------> | 1 |
                +---+

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and during for $x (9) { ... 
                +---+
$MAIN_X  -----> | 9 |
                +---+
              
                +---+
$FOO_X  ------> | 1 |
                +---+

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So calling foo from within the loop causes the 1 to be printed, not the 9.

You are mostly right about how localization and for aliasing work, expect that its not entire values that are temporarily saved, it is simply a C pointer, either in the symbol table (eg local $x), in the scratchpad (eg my $x; for $x ()), or in an array or hash slot (eg local $h{foo}), that is pushed onto perl's savestack and replaced with a pointer to a different value.

Dave.

In reply to Re^3: Closures & aliases by dave_the_m
in thread Closures & aliases by BrowserUk

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