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

my $str = <<EOF;
<students>
    <student>
        <name>John</name>
        <id>001</id>
        <gpa>A</gpa>
    </student>
    <student>
        <name>John</name>
        <id>002</id>
        <gpa>C</gpa>
    </student>
</students>
EOF

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

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

__END__

<students>
  <student>
    <name>John</name>
    <id>001</id>
    <gpa>A</gpa>
  </student>
  <student>
    <name>John</name>
    <id>002</id>
    <gpa>B</gpa>
  </student>
</students>