Manipulating reference to array of array references

[Mohan L]

Dear all,

#!/usr/bin/env perl
#dummy.pl
use strict;
use warnings;
use Data::Dumper;

my $ref_to_AoA = [
        [ "fred", "barney",undef,"pebbles", "bambam", "dino", ],
        [ "homer",undef,"bart",undef, "marge", "maggie", ],
        [ "george", "jane",undef, "elroy",undef,"judy", ],
    ];
    print Dumper $ref_to_AoA;
    print $ref_to_AoA->[0]->[0];


[mohan@localhost ~]$ ./dymmy.pl
$VAR1 = [
          [
            'fred',
            'barney',
            undef,
            'pebbles',
            'bambam',
            'dino'
          ],
          [
            'homer',
            undef,
            'bart',
            undef,
            'marge',
            'maggie'
          ],
          [
            'george',
            'jane',
            undef,
            'elroy',
            undef,
            'judy'
          ]
        ];

To print 'fred' , I can use like : print $ref_to_AoA->[0]->[0];

But What I want is, I want to replace all 'undef' to a string 'foo'.  What
is the efficient way I will replace it (the array is larger then what I am
showing above). Any help and explanation will be appreciated.

Thanks & Rg
Mohan L