如何使用Perl将冒号放入字符串中最后两个字符？

my $string = "Hello";
my $colon_string = $string . ":" . $string;
print $colon_string;

my $string = "Hello";
my $colon_string = substr($string, 0, -2) . ":" . substr($string, -2);
print $colon_string;

my $string = "Hello";
my $colon_string = $string =~ s/(.{2})$/$1:/r;
print $colon_string;