my $fitness = sub { my ($key, $hashr ) = @_ ;
  return .3 <= $key && . 7 >= $key && $hashr->{$key} =~ m/foo/i ? 1 : 0
}
my @good_keys= find_good_keys( \%hash, $fitness);

sub find_good_keys {
  my ( $hashr, $fitness ) = @_;
  return map { $fitness->( $key, $hashr ) } keys %$hashr;
}