my %hash = ( .1 => 'url1', .2 => 'url2', .5 => 'url5', .9 =>'url9' );
my $fitness = sub { $_ > .4 };
my @good_keys = find_good_keys( \%my_hash, $fitness );

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