# If your $key is *not* what we're looking for, remove i
+t from the hash
   if ( !grep ($key, @keysToKeep) )
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well what you want something like Python's in operator, but the Perl way to check inclusion within a set is to use hashes.

so if ($keysToKeep{$key}) would do it most efficiently.

There is a new smart-match operator ~~ for in, but thats still somehow experimental

  DB<137> 1 ~~ [1,2,3]
 => 1

  DB<138> "a" ~~ [1,2,3]
 => ""

  DB<139> "4" ~~ [1,2,3]
 => ""

  DB<140> "2" ~~ [1,2,3]
 => 1
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edit

Anyway using a hash-slice is the fastest way to filter keys:

  DB<145> %h=(a=>1,b=>2,c=>3)
 => ("a", 1, "b", 2, "c", 3)

  DB<146> @keysToDelete=qw/b c d/
 => ("b", "c", "d")

  DB<147> delete @h{@keysToDelete}
 => (2, 3, undef)

  DB<148> \%h
 => { a => 1 }
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update

or

  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 }
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but please notice how key "e" jumped into existence now, which is no problem if you use a guarantied subset.