If the file is not "too big" (e.g. mega gigabytes?) you can just use XML::LibXML:

my $x = XML::LibXML->new;
my $doc = $x->parse_file($file);
my $root = $doc->getDocumentElement();

my ( $name, $content ) = qw(@name @content);

for my $att (
  $root->findnodes(qq[
    //package/metadata/x-metadata/meta[$name='dtb:totalTime']/$content
  ])
) {
  my $old_val = $att->value();
  print "Changing value from $old_val to 345\n";
  $att->setValue(345);
}
eval { $doc->toFile($file, 1) } or die "Could not write to $file: $@";
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