use warnings;
use strict;
use Data::Dump;

my %expensive_lookup = (
		math01 => { instructor=>'alice' },
		sci99  => { instructor=>'bob'   },
		gym55  => { instructor=>'carol' },
		cs101  => { instructor=>'larry' },
	);

my %students = (
	sid01  => [qw/ math01 gym55 /],
	sid007 => [qw/ gym55 sci99  /],
	sid42  => [qw/ cs101 math01 /],
);

my %catalog = ();

for my $student_id (keys %students) {
	for my $course_number ( @{ $students{$student_id} } ) {
		if ( not exists $catalog{$course_number} ) {
			$catalog{$course_number} =
				$expensive_lookup{$course_number};
			$catalog{$course_number}{course_number} = $course_number;
		}
		$course_number = $catalog{$course_number};
	}
}

dd \%catalog, \%students;

__END__

do {
  my $a = {
    cs101  => { course_number => "cs101", instructor => "larry" },
    gym55  => { course_number => "gym55", instructor => "carol" },
    math01 => { course_number => "math01", instructor => "alice" },
    sci99  => { course_number => "sci99", instructor => "bob" },
  };
  (
    $a,
    {
      sid007 => [$a->{gym55}, $a->{sci99}],
      sid01  => [$a->{math01}, $a->{gym55}],
      sid42  => [$a->{cs101}, $a->{math01}],
    },
  );
}