sub ReadSource( $ $ )
{
    my $DataSource = shift;
    my $HashRef    = shift;
    
    foreach my $ItemID (@Data)
    {
        my $data_array_ref = $HashRef->{$ItemID};

        if ( $data_array_ref )
        {
            push( @{$data_array_ref}, $DataSource );
        }
        else
        {
            $HashRef->{$ItemID} = [ $DataSource ];
        }
    }
}

##</code><code>##

  my %myhash = ();  # empty hash
  my @Data = ( 'this', 'that', 'that over there' );

  my $datasource = 'banana';

  ReadSource( $datasource, \%myhash );

##</code><code>##

  use Data::Dumper;

  ReadSource( $datasource, \%myhash );
  print( Dumper( \%myhash ) . "\n" );

##</code><code>##

$VAR1 = {
          'this' => [
                      'banana'
                    ],
          'that' => [
                      'banana'
                    ],
          'that over there' => [
                                 'banana'
                               ]
        };