Depends on how and where you're building it; as was mentioned give us a sample and someone can comment on specifics. That being said though here's a way to (for example) initialize your hash from a file with one item per list lazily and only one time (unless you explicitly clear it):

if( exists _get_cache()->{ $candidate } ) {
  say qq{IT DOES};
} else {
  say qq{No such luck . . .};
}

{ ## Block to scope our cache to just these subs
  my $lookup_cache = undef;

  sub _reset_cache { $lookup_cache   = undef;         }
  sub _get_cache   { $lookup_cache //= _load_cache(); }

  sub _load_cache {
    ## presuming you've declared file var somewhere above . . .
    open( my $fh, q{<}, $CACHE_FILE_NAME ) 
      or die qq{Can't open '$CACHE_FILE_NAME': $!\n};

    $lookup_cache = {};

    while( <$fh> ) {
      chomp;
      $lookup_cache->{ $_ } = 1;
    }

    close( $fh );

    return $lookup_cache;
  }
} ## End of limited scope block.
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