In addition to what others have said, you could do: 
my @to_keep = (...);
my %keepers;
@keepers{@to_keep} = ();

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which you would then test with exists $keepers{$x}. Althought I'm curious why you need the %keepers hash if you have the @to_keep array.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: Hash assignments using map by japhy
in thread Hash assignments using map by njcodewarrior

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