You won't see the repeated items, like major, since the hash will only hold the last value.

$ perl -d
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

$students{"Mary Pitts"}=(
        {
                "year"=>"4",
                "GPA"=>"4",
                "major"=>"Economics",
                "major"=>"Economics",
                "email"=>"mpitts\@school.edu"
        }
);

print "Mike was here...";
main::(-:1):    $students{"Mary Pitts"}=(
main::(-:2):            {
main::(-:3):                    "year"=>"4",
main::(-:4):                    "GPA"=>"4",
main::(-:5):                    "major"=>"Economics",
main::(-:6):                    "major"=>"Economics",
main::(-:7):                    "email"=>"mpitts\@school.edu"
  DB<1> n
main::(-:11):   print "Mike was here...";
  DB<1> x %students
0  'Mary Pitts'
1  HASH(0xa038a8c)
   'GPA' => 4
   'email' => 'mpitts@school.edu'
   'major' => 'Economics'
   'year' => 4
[download]

See? Only one major...

Are you saying you want an error to occur when a duplicate value is attempted? A large debate occurred on p5p recently (a few months ago) about clamped hashes, but I don't remember what the outcome was, or whether the feature will be in 5.8.
--
Mike