s/\[\[(\w+)\]\]/$helper->($1)/eg;

##</code><code>##

panic: sv_pos_b2u: bad byte offset at [...].

##</code><code>##

s/\[\[(\w+)\]\]/@{[$helper->($1)]}/g;

##</code><code>##

while (m/\[\[(\w+)\]\]/) {
    my $foo = $helper->($1);
    s/\[\[(\w+)\]\]/$foo/;
}