As 'jfroebe' said below, xpdformat is probably also writing to STDERR (which is *NOT* captured by backticks). Read about STDERR by doing

   perldoc -q STDERR
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at a command prompt. The perldoc '-q' says to search the perldoc documentation for string "STDERR". Here's a snippet from that:

Found in C:\Perl\lib\pod\perlfaq8.pod
  How can I capture STDERR from an external command?
    There are three basic ways of running external commands:

        system $cmd;                # using system()
        $output = `$cmd`;           # using backticks (``)
        open (PIPE, "cmd |");       # using open()

    With system(), both STDOUT and STDERR will go the same place as th
+e
    script's STDOUT and STDERR, unless the system() command redirects 
+them.
    Backticks and open() read only the STDOUT of your command.

Here's another snippet:
-----------------------

    You can also use file-descriptor redirection to make STDERR a dupl
+icate
    of STDOUT:

        $output = `$cmd 2>&1`;
        open (PIPE, "cmd 2>&1 |");

And another snippet:
--------------------

    ...To capture a command's STDERR and STDOUT together:

        $output = `cmd 2>&1`;           # either with backticks
        $pid = open(PH, "cmd 2>&1 |");  # or with an open pipe
        while (<PH>) { }                #    plus a read
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Anyway, hope that helps.