I see solutions to your problem, but since I don't see any explanation, I'll provide one.

Your problems lies with the fact that hashes can only have a single value at any given key. However, those value can be an array (or hash) reference which may contain a list of items.

One provided solution provided is:
push @{$fruits{$a}},$b
which is equivalent to:
$fruits{$a} = [] unless $fruits{$a}; # create array, save ref
push @{$fruits{$a}},$b

People suggested you could also form a string representing the list of values:
if (defined($fruits{$a})) {
   $fruits{$a} .= ','.$b;  # Add to existing list
} else {
   $fruits{$a} = $b;       # Start new list
}
This isn't good if you're planning on manipulating the individual values, but it good if you're just going to print out the list without modification.

One of the Perl docs refers to Array of Arrays (AoA). You should read up on those.