## your code is effectively this:
   my %hash_list = ( " Bill 34", " Jane 28", " John 20", " Tom 19", " Wall 18" );

   ## which means...
   my %hash_list = (
      # key         # value
      " Bill 34" => " Jane 28",
      " John 20" => " Tom 19",
      " Wall 18  => undef
   );

##</code><code>##

   print sort ( split( /,/, $names ) );

##</code><code>##

    my $names    = "John 20, Bill 34... etc";
    my @elems    = split( /\s*,\s*/, $names );
    my %num2name = map{ reverse split( /\s/, $_ ) } @elems;

    my @sorted   = map{ $num2name{$_} } sort { $a <=> $b } keys %num2name;
    print "@sorted\n";