There's two possible and different ways to do this.

push @{$fruits{$a}},$b
[download]

will create a hash-of-arrays where each hash element references an array.

$fruits{$a} .= ','.$b ;
[download]

will just concatenate all values($b) for a particular key ($a).
The selection of one of these depends entirely on what is to be done with the hash after it has been populated. For computational purposes, dealing with a hash-of-arrays is (IMHO) easier than doing a series of splits on a value which was constructed by concatenating...
The hash-of-arrays is easier to iterate through, whereas if you just need to print to output, concatenating might be a better option.

Manav