use strict;
use XML::Simple;

$filename = "yours.xml";
xml_edit($filename); # return done 1 || error 0

sub xml_edit($){
my$bool=0;
%DB=load($_[0]);
if(exists$DB{student}{id}{2})
{
$DB{student}{gpa}='C';
$bool++;
}
save($_[0],\%DB);
return$bool;
}

#untested