Scoping issues with code evaluation asserstions?

[Cirollo]

I'm trying to use a code evaluation assertion in a regex inside of a subroutine, and I'm seeing this strange behavior where it only executes the code the first time that I call the subroutine. Here is the simplest example I could come up with:

sub regex {
    my $in = shift;
    my $ret = '';

    # Assign something to $ret in a code assertion
    $in =~ m/^(a)(?{$ret=1})/;

    return $ret;
}

while(<DATA>) {
    print regex($_) . ".";
}

__DATA__
a
b
a
abcd
bcda

The output that I get is "1...." when I would expect to get "1.1.1.1.1." As I understand it, the value 1 should get assigned to $ret every time through the regex, regardless of whether ^(a) matched. Can anyone explain this? I'm rather confused.