If you don't mind reformatting the data slightly you can run a quick regex across the data to reverse the order of the alphanumeric and numeric data.

chomp(my @unsorted = (<DATA>));
map{ $_ =~ s/(\d+),(\w+)/$2,$1/ } @unsorted;
my @sorted = sort { $a cmp $b } @unsorted;
say for @sorted;
[download]

Or do you want to do a complex sort without reformatting? In which case you can do something like the below, which will sort without reformating.

chomp(my @unsorted = (<DATA>));
my @sorted = sort { 
   (split(',',$a))[1] cmp (split(',',$b))[1] ||
   (split(',',$a))[0] cmp (split(',',$b))[0]
   } @unsorted;
say for @sorted;
[download]

But again, if your ultimate goal is simple sums of each alphanumeric code, there is no need to sort unless you are outputting each sum as you go and purging it from memory. If holding all the sums in memory at one time is not a problem, summing into a hash should be all you need as I did in my previous example.

#!/usr/bin/perl -w
use strict;
use List::Util qw(sum);

my %Alpha2Number;  #Hash of Array  def=>[406744,107297]

while (<DATA>)
{
    chomp;
    my($number, $ltrs) = split(',',$_);
    push @{$Alpha2Number{$ltrs}},$number;
}

print "Totals:\n";
foreach my $alpha (sort keys %Alpha2Number)
{
   print sum (@{$Alpha2Number{$alpha}}), ",$alpha\n";
}

print "\nSorted Data:\n";
foreach my $alpha (sort keys %Alpha2Number)
{
   print "$_,$alpha\n" for @{$Alpha2Number{$alpha}};

   #use this line if you want also to sort the numbers
   #print "$_,$alpha\n" for sort{$a<=>$b} @{$Alpha2Number{$alpha}};
}

=PRINTS
Totals:
1918297,abc
514041,def
382404,mps
335563,rxwz

Sorted Data:
1208782,abc
367455,abc
226159,abc
115901,abc
406744,def
107297,def
283191,mps
99213,mps
197688,rxwz
137875,rxwz

=cut


__DATA__
1208782,abc
406744,def
367455,abc
283191,mps
226159,abc
197688,rxwz
137875,rxwz
115901,abc
107297,def
99213,mps
[download]