I have read numerous articles that detail how it is not possible to have a PEEK at STDIN (if it is linked to a pipe). The typical suggestion - which I would implement, given different circumstances - is to use a read-ahead buffer and redo a loop iteration, reusing the buffer.

My problem is now that I would like to use an XML parser (XML::Twig) if, and only if, the first line of the input contains a valid XML header.

So a read-ahead buffer is not doing me much good if XML::Twig does not accept an incomplete XML file (missing the header) in its "parsefile" function.

Basically, I was hoping to do it like this: 
   my $header = peek(<>);

   if (is_xml ($header)) {
      my $t = XML::Twig->new();
      $t->parse (\*STDIN);
   }
   else {
      # do something else with <>
   }

[download]

Any suggestions on how to tackle this?

In reply to peek at STDIN, to determine data type and then pass STDIN to a parser by aral

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