But to use just a single 2D array. $array[0][1] is the second element of the first row. If you have a reference to that row, then: $this_row_ref->[1] is the way. $row_ref->[-1] the -1 index means "last element".

A 2D array is an array of references to arrays. All multi-dimensional structures are references until you get to the last dimension.

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
use Data::Dump qw(pp);

######  some data to work with
###### 
                #id     #name
my @clients = ( [123 , 'clientA'], #using this instead of SQL query
                [346 , 'clientB'],
                [789 , 'clientC']);
               
               
                     #id  #integer
my @int_numbers =  ( #[123, 33]   #using this instead of SQL query
                      [346 , 1],
                      [789 , 2] );  
                      
######
######
                
my @Table2D;  #using an Array of Array 
                           
foreach my $row_ref  (@clients)
{
   my ($id, $name) = @$row_ref;
   
   #you get this from the first query
   
   push @Table2D, [$id,$name];
}

print pp (\@Table2D), "\n";
# now the 2D array is started...
#
#[[123, "clientA"], [346, "clientB"], [789, "clientC"]]

#now expand each row by one column
foreach my $row_ref  (@Table2D)
{
   push @$row_ref, 0;  
}

print pp (\@Table2D), "\n";
#[[123, "clientA", 0], [346, "clientB", 0], [789, "clientC", 0]]

#now double loop ...
#if we use a Hash of Array, we don't need this...
#of course this is super inefficient 

foreach my $query_ref (@int_numbers)
{
   # you get these from second query..
   # 
   my ($id2, $integer_number) = @$query_ref;
   
   foreach my $row_ref (@Table2D)
   {
      my ($id_in_table) = @$row_ref;  #just the first column!
      
      if ($id_in_table == $id2)  #searching table for the id
      {                          #in query 2.
         $row_ref->[-1] = $integer_number;
         last;                     #found... stop looking
      } 
   }
}

print pp (\@Table2D), "\n";
# see now clientB and clientC got their last column updated
# but clientA did not!
#[[123, "clientA", 0], [346, "clientB", 1], [789, "clientC", 2]]
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