use feature qw/say/;
use strict;
use warnings;

my $string = "afdhauio af ad a reason one reason two 1 2 3's reason for,reason7,reason8 ejroapjr";

$string =~ m/
    (?:reason.*?){2}    # Skip the first two reasons
    (reason.*)          # Keep the third and all text after it.
    $                   # To end of string.
/x or die "No match.\n";
say $1;