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

$str =~ s/<student>(.*?)<\/student>/check_record($1)/gse;

print $str;

sub check_record
{
    my $rec = shift;
    $rec =~ s/(<gpa>\s*)C(\s*<\/gpa>)/$1B$2/ if($rec =~ /<id>\s*002\s*<\/id>/);
    return "<student>$rec</student>";
}