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 :

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The foreach modifier is an iterator: For each value in EXPR, it aliases $_ to the value and executes the statement

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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,

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(m//) ... in a list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2, $3...)

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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.