Hi Perlian, How come my code below doesn't update the original hash (%line)? 
my %line = ('A' =>1,
            'B' =>1,
            'C' =>1);

my @tojoin = qw (W X Y Z);


foreach  my $line ( keys %line  )
{
   print "$line\n";

   foreach my $tojoin ( @tojoin  )
   {
      my $nstr = $line.$tojoin;
      $line{$nstr} = 1;
   }           
     last if ($line eq 'AYW');

}

[download]
Such that finally the %line will give: 
A => 1,
B => 1,
C => 1,
AW => 1,

.... until

AYW => 1

[download]

In reply to Updating A Hash Recursively by Anonymous Monk

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