I'll take a crack at this one, since it gave me the same kind of problems when I started using regexps as loop controls. Using brother frankus' example :
$_='axxbcaxbagaxbacba';
foreach my $a (m/a(.*?)b/g) {
    print "$a\n";
}

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OK, so, from perlman :
The foreach modifier is an iterator: For each value in EXPR, it aliases $_ to the value and executes the statement
So, right away, we see that foreach doesn't care about, or understand $1 and company at all. It only knows about its control variable ($a, in this case) and its list : (m/a(.*?)b/g).
So what's the for statement's list? It's generated from m/a(.*?)b/g. And, as perlop states,
(m//) ... in a list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2, $3...)
So, with this regexp, you get an anonymous list of 4 elements. It's like writing 
foreach my $a ($1, $2, $3, $4) {

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by the time the code in the loop's running, the regexp has run and returned a list of values that foreach will process.
I hope that makes things a little clearer, this gave me trouble for a while, and hopefully this explanation will minimize the trouble it gives you.
update see below... sometimes convenient variables aren't good for practical use.

In reply to (boo) Using regexps as lists in loops by boo_radley
in thread $1 doesn't reset? by alfie

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