Try this see if it helps.

#!/usr/bin/perl -w
# gene.plx
# use strict;

my %hash = ();
while (<DATA>) {
    chomp($_);
    my ($dept,$email)=split(/:/);
    $hash{ $dept } .= $email.',';
}
s/,\z// for values %hash;

print "\n";

while ( my ($key, $value) = each(%hash) ) {
      print "$key => $value\n";
      }

print "size of hash:  " . keys( %hash ) . ".\n";
print '-' x (60),"\n"; 


__DATA__ 
6:car
1:human
3:fruits
9:flower
2:flower
5:human
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