toolic was right, the problem is that extra question mark: 
my ($data_in, $data_out);

$data_in = q|<container><a>smile!!! </a><b> smile again!! </b></contai
+ner><?query?>|;

while ($data_in =~ /<container><a>(.+?)<\/a><b>(.+?)<\/b><\/container>
+(<\?query\?>)?/ig)  {
  $data_out = "$2, $1$3 ";  #errors here
  ; #further processing here 
  print $data_out;
}

[download]

If you put back the second question mark at the end you will get the error: 

Use of uninitialized value $3 in concatenation (.) or string at perlmo
+nks_700143.pl line 8.

[download]
If you are new to using regular expressions and are on Windows, you can try this free tool: Regex Coach. It will help you see, graphically, how the regex breaks down and will allow you to step through your expression.

Celebrate Intellectual Diversity

In reply to Re: Special Variables in Regular Expression by InfiniteSilence
in thread Special Variables in Regular Expression by fiat

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