DB<153> %h=(a=>1,b=>2,c=>3)
 => ("a", 1, "b", 2, "c", 3)

  DB<154> @keysToKeep=qw/a e/
 => ("a", "e")

  DB<155> %h2=()

  DB<156> @h2{@keysToKeep}=@h{@keysToKeep}
 => (1, undef)

  DB<157> \%h2
 => { a => 1, e => undef }