Looks like you need to create a new table with the join; the following seems to work ok for me:

#!/usr/bin/perl

use strict;
use warnings;

use Data::Table;

my $t1 = new Data::Table( [ [ 1, 2, 3   ], [ 10, 20, 30    ] ], [ 'A',
+ 'B' ], 1 );
my $t2 = new Data::Table( [ [ 1, 2, 3 ,4], [ 11, 22, 33,44 ] ], [ 'C',
+ 'D' ], 1 );

print $t1->csv;
print "-------\n";
print $t2->csv;

print "-------\n";
my $t3 = $t1->join( $t2, 3, ["A"],["C"] );
print $t3->csv;
exit;
[download]

Good luck - and thanks for bringing Data::Table in sight, seems a nice module to play with that I didn't know so far.