use Data::Dump qw(dump); 

my %hash = (
  key1 => 'Value1',
  key2 => 'Value2',
  key3 => 'Value3',  );

print "old hash is " . dump(\%hash) ."\n";
map { s/2/_two/; } %hash;
print "new hash is " . dump(\%hash) ."\n";

# output is:
# old hash is { key1 => "Value1", key2 => "Value2", key3 => "Value3" }
# new hash is { key1 => "Value1", key2 => "Value_two", key3 => "Value3" }