my %dict = map {split /\s/,$_,2} <$d>;
close $d;

while (defined (my $line=<$w>)){
  for (split /\s/,$line){
    if (my $meaning = $dict{$_}){
       print "$_: $meaning\n";
       next;
    }
    print "$_ is not in the dictionary\n";
  }
}
[download]

I would write that as:

my %dict = map split( ' ', $_, 2 ), <$d>;
close $d;

while ( <$w> ) {
  for my $word ( split ) {
    if ( exists $dict{ $word } ) {
       print "$word: $dict{ $word }";
       next;
    }
    print "$word is not in the dictionary\n";
  }
}
[download]

The use of /\s/ with split will screw up the dictionary if there are any leading whitespace.    (Anything that can go wrong, will go wrong. -- Murphy)