Hey all, I ended up using XML::Twig for removing and re-adding the appropriate filenames. My code looks something like this:

my $t = new XML::Twig( ignore_elts => {'/bxh/datarec/filename' => 1,
                       '/bxh/datarec/fileoffset' => 1,
                       '/bxh/datarec/filerecordsize' => 1,
                      },
               TwigHandlers =>
               {
            '/bxh/datarec' => \&add_filenames,
               },
               pretty_print=>'indented',
             );

$t->parsefile($file);      # build it
$t->print;                 # output the twig


sub add_filenames {
  my($t,$drec) = @_;

  my @filenames = glob "mri*.img";
  foreach my $file(@filenames) {
    my $elt = new XML::Twig::Elt('filename',$file);
    $elt->paste('last_child',$drec);
    my $size = -s $file;
    $elt = new XML::Twig::Elt('fileoffset','0');
    $elt->paste('last_child',$drec);
    $elt = new XML::Twig::Elt('filerecordsize',$size);
    $elt->paste('last_child',$drec);
  }
}
[download]

Thanks for everyone's input!