my $ref_a = [
             [ qw/a b c/ ],
             [ qw/d e f/ ],
             [ qw/g h i/ ],
            ];

my $ref_b = [
             [ qw/j k l/ ],
             [ qw/m n o/ ],
             [ qw/p q r/ ],
            ];

for my $i (0..$#$ref_a) {
    push @{$ref_a->[$i]}, @{$ref_b->[$i]};
}