What have you tried so far?

Think of a way how you'd do it by hand, then write a Perl program that does the same.

For XML manipulation, people often recommend XML::LibXML or XML::Twig.

my $input := open 935956.xml ;
my $student_hash := hash concat(@name, '#', @number) $input/xml/data/s
+tudent ;
my $semister_hash := hash ../@number $input/xml/data/semister/@subject
+_names ;

my $out := create xml ;
for my $university in $input/xml/university {
    my $new_uni := cp $university into $out/xml ;

    for my $student in $new_uni//student {
        cp xsh:lookup('student_hash', concat($student/@name, '#', $stu
+dent/@number))/@address into $student ;
        for $student/semester cp xsh:lookup('semister_hash', ./@number
+) into . ;
    }
}

save :f 935956-out.xml $out ;
[download]

thanks its working