my %courses;
my $mnemonic;  # this is the correct spelling :)

while (<COD>) {  # let's use $_, shall we?
   next unless ( /\S/ );  # skip blank lines;
   my @fields;

   my ($id, $rest) = unpack("xA5xA*", $_);  # break line into 2 pieces

   if ( $id =~ /^\d{5}$/ ) { # it's the start of a record
      ($mnemonic,@fields = unpack("A4xA4xA4xA2xA2xA28A*", $rest);
      # work out what to do with @fields; $mnemonic will retain
      # it's current value till the next one is encountered,
      # so sub-records after this one will be added to the
      # correct hash element.
   }
   elsif ( $rest =~ /^\d+-\d+/ ) { # it's a sub-record
      my ($time,$days,$bldg,$room,$end) = unpack("A9xA6xA4xA4xA*", $rest);
      # you need to work out what to do with $end,
      # and push stuff into the current $courses{$mnemonic} structure
   }
   else {
      # do something else with (or ignore) other stuff
   }
}