This leads to possibly ugly output as there will be an extra comma in the output

Also will a value appear more than once, and should the values be sorted as well? Im thinking a HoH might be overkill, but another (better?) way to do it.

my %all;
while (<>) {
   s/\s+//g;
   my($k, $v) = split /,/;
   next unless defined $k and defined $v;
   $all{$k}{$v}++;
}

for ( sort keys %all ) {
   print "$_, ", join(', ', sort keys %{ $all{$_} }), "\n";
}
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