sub regex {
    my $in = shift;

    # Assign something to $ret in a code assertion
    my $function = (caller 0)[3];
    no strict 'refs';
    $$function = '';
    $in =~ m/^(a)(?{ $$function = 1})/;

    return $$function;
}

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

__DATA__
a
b
a
abcd
bcda