use strict;
use warnings;
use feature 'say';
use PDL;

use Time::HiRes 'time';
my $t = time;

use constant MAX => 1e6;
use constant TOP => MAX < 20 ? MAX : 20;

my $seqs = 1 + sequence( longlong, MAX );
my $lengths = ones( short, MAX );

while ( any my $good_mask = $seqs-> inplace
                                 -> setvaltobad( 1 )
                                 -> isgood ) {
    my $odd_mask = $seqs & 1;
    $lengths-> where( $odd_mask ) ++;
    $lengths-> where( $good_mask ) ++;
    ( $seqs-> where( $odd_mask ) *= 3 ) ++; 
    $seqs >>= 1; 
}

my $top_i = $lengths-> qsorti
                    -> slice([ MAX - 1, MAX - TOP ]);

say $lengths-> index( $top_i )
            -> longlong
            -> cat( $top_i + 1 )
            -> transpose;
say time - $t;

__END__

[
 [   525 837799]
 [   509 626331]
...
 [   445 938143]
 [   445 906175]
 [   445 922525]
 [   445 922526]
]

7.98023009300232