Another variation is to use a HoA (Hash of Array) to store the data. Then cycle through the sorted keys and either
a) print sum of array values for totals, or
b) print the array values themselves

#!/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
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