while(<DATA>){
chomp;
  $hash{$_}++ for split;  
}
##</code><code>##

while(<DATA>){  #get a line of text from the DATA area, put it in special variable $_
    chomp;      #remove the "new line" character from the $_ variable
    #create a word array from the $_ variable
    #split with no options splits on whitespace by default
    @words = split;
    foreach $word (@words) {
        $hash{$word}++;  #increment the value pointed to by the $hash{$word} "key"
    }  
}

##</code><code>##

board => 1
chalk => 1
class => 2
desk => 1
professor => 1
school => 2
students => 2
table => 1
teacher => 3

##</code><code>##

foreach $word (sort keys %hash) {
    print "$word\n";
}
print "\n";
foreach $word (sort keys %hash) {
    if ($hash{$word} > 1) {
        print "$word\n";
    }
}