use warnings;
use strict;
use XML::Twig;

open File, 'test.xml';
sysread(FILE, my $str, -s FILE);

my $t = XML::Twig->new(
        twig_handlers => {
            student => \&student,
        },
        pretty_print  => 'indented',
);
$t->parse($str);
$t->print($str);
print "\n";

close FILE;

sub student {
    my ($t, $elt) = @_;
    if ($elt->field('id') eq '002') {
        $elt->first_child('gpa')->set_text('B');
    }
}