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1 while s/<([^<]+?)>([^<]*?)<\/>/
   $_{$1}->{$2} ||= do {
...
      $func->($2)
   }
/e;

my %cb = (
   use => \&use,
...
   #TODO: Make sure $cb->{$1} exists.
   $_{$1}->{$2} ||= $cb->{$1}->($2)
/e;