Rewriting it to use pos() like this works for me: 
while (m/\[\[\w+\]\]/) {
    my $oldpos = pos;
    m/\[\[(\w+)\]\]/;

    my $expansion = $helper->($1);
    pos = $oldpos;
    s/[[$1]]/$expansion/;
}

[download]
It's a little nasty due to matching more times than I'd like, but it works. :)

In reply to Re: Functions in substitutions (s///) and Perl 5.8. by teichman
in thread Functions in substitutions (s///) and Perl 5.8. by v_thunder

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