gistfile1.pl

#!/usr/bin/perl
use Benchmark qw(:all);
my @strings = (123456, 1234567890, 1234567890123456);

foreach my $str (@strings) {
    print "\nString of " . length($str) . " characters\n";
    cmpthese(
        1000000,
        {
            'length and substr' => sub {
                'X' x (length($str) - 4) . substr($str, -4);
            },
            'regex w/replace' => sub {
                $str =~ s/(\d+)(\d{4})/('X' x length $1) . $2/e;
            },
            'regex' => sub {
                $str =~ s/\d(?=\d{4})/X/g;
            },
        }
    );
}

=results

String of 6 characters
                        Rate length and substr            regex  regex w/replace
length and substr  2127660/s                --             -21%             -83%
regex              2702703/s               27%               --             -78%
regex w/replace   12500000/s              487%             362%               --

String of 10 characters
                        Rate length and substr            regex  regex w/replace
length and substr  2222222/s                --             -16%             -84%
regex              2631579/s               18%               --             -82%
regex w/replace   14285714/s              543%             443%               --

String of 20 characters
                       Rate  regex w/replace             regex length and substr
regex w/replace    775194/s               --              -51%              -62%
regex             1587302/s             105%                --              -22%
length and substr 2040816/s             163%               29%                --
=cut