#define your data:
my @hash = ( 
  .1 => 'url1',
  .2 => 'url2',
  .3 => 'url3',
  .7 => 'url7',
  .9 => 'url9',
);
#define the fitness function / match determination
my $good_value = sub { $_ > 0.4 };

#find the keys that match the fitness function:
my @good_keys = map { $good_value->($_) } keys @hash;

#now do something with the keys:
##eg. print the values, in hash order:
for my $key (@good_keys)
{
   print $hash{ $key} , "\n";
}
## grab just the urls from the good keys, without including the keys.
my @good_values = @hash{ @good_keys };

##e.g. save a hash with just the subset of new keys
my %good_hash;
@good_hash{ @good_keys } = @hash{ @good_keys };