use strict;
use warnings;
use Data::Dump qw(pp);
$|=1;

my %hash = ( 'a' => 2,
             'b' => 3,
             'c'=> 4,
             'd' => 5);
             
my %key_map = ('a'=> 'abc',
               'd'=> 'def');
               
pp \%hash;
pp \%key_map;

############# added code #########
print "Perl size of 'hash' ", scalar(%hash),"\n";


foreach my $key (keys %hash)
{
   if (defined (my $new_key = $key_map{$key}))
   {
      print "New key for $key is $new_key\n";
      my $current_value = $hash{$key};
      delete $hash{$key};
      $hash{$new_key} = $current_value;
   } 
}
print "result of hash mods:\n";
pp \%hash;

__END__
Prints:
{ a => 2, b => 3, c => 4, d => 5 }
{ a => "abc", d => "def" }
Perl size of 'hash' 3/8
New key for d is def
New key for a is abc
result of hash mods:
{ abc => 2, b => 3, c => 4, def => 5 }