xgettext: Perl: Avoid unbounded nesting_depth growth, part 2.

xgettext: Perl: Avoid unbounded nesting_depth growth, part 2.

* gettext-tools/src/x-perl.c: Return from extract_balanced when a subroutine's
definition terminates.
(extract_balanced): Test whether the first token is 'sub'. If so, don't use
the usual symbol parsing and return when a token_type_lbrace was seen.

Test cases:
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sub import {
}
==========================================
sub import {
  if (!$module_loaded) {
    Texinfo::XSLoader::override("", "");
    Texinfo::XSLoader::override("", "");
    $module_loaded = 1;
  }
  goto &Exporter::import;
}
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