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1 #!/usr/bin/perl -w
  2 
...
 20 print "\nEnter Key: ";
 21 my  $type = <stdin>;
 22 print  "you will be taking $course{$type} this semester\n";

:~/pbe/chap5$ ./exer5.pl 
4 => perl
...
Enter Key: 2
Use of uninitialized value within %course in concatenation (.) or stri
+ng at ./exer5.pl line 22, <stdin> line 1.
you will be taking  this semester