use strict;
use warnings;

use Data::Dumper;

my %hash = 
   map { $_->[0] => join q{,}, @$_[1, 2] }
   map { chomp; [ split m{\s*,\s*} ] }
   <DATA>;

print Dumper \%hash;

__END__
A, B, C
D,E,F
G, H,I

##</code><code>##

$VAR1 = {
          'A' => 'B,C',
          'D' => 'E,F',
          'G' => 'H,I'
        };