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Global symbol "$node_c" requires explicit package name (did you forget
+ to declare "my $node_c"?) at derp.pl line 13.
Execution of derp.pl aborted due to compilation errors.

use strict;
use warnings;
...
my($object0) = $doc->findnodes("/header/id/c/text()");
$object0->setData( $node_c );
print $doc;

Please specify node c content
...
      <c>derp</c>
   </id>
</header>

#!/usr/bin/perl
use strict;
...
chomp( my $node_c = <STDIN> );
$dom->at('id > c')->content( $node_c );
say $dom->content;

Current value of c: NA
Please specify node c content: 
...
      <c>derp</c>
   </id>
</header>