Thanks all gurus! I admit language is a barrier to communicate with monks,especially for a perl beginner:)
why

print oct(0xff); # 173
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? because oct function require a "string" parameter,So perl convert(stealthily) 0xff(number) to 255(string),then oct convert 255(octal) to 173(decimal). right?I hopefully don't misunderstand reply being provided.
But you have to admit it's a trap to a beginner.Although I add "use warnings" and" use strict",perl don't give any useful messages.It just return 173. For me,I prefer to choose print(printf):

my $num = 0xff;
print oct($num); #wrong!
printf('xo',$num);# right!
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p.s. maybe I can use pack,but it's another issue.