use YAML::Syck qw( Dump );

my %orig = qw( a 1 b 2 c 3 d 4 e 5 f 6 );
my %new;
{
  my @keys;
  @new{ @keys } = @orig{ @keys = grep /[adf]/, keys %orig };
}

print Dump( { orig => \%orig, new => \%new } ), "\n";

exit 0;

__END__
--- 
new: 
  a: 1
  d: 4
  f: 6
orig: 
  a: 1
  b: 2
  c: 3
  d: 4
  e: 5
  f: 6