Perl 使用constant注意不能expand常數

As with all use directives, defining a constant happens at compile time. Thus, it's probably not correct to put a constant declaration inside of a conditional statement (like if ($foo) { use constant ... } ).

Constants defined using this module cannot be interpolated into strings like variables. However, concatenation works just fine:

    print "Pi equals PI...\n";        # WRONG: does not expand "PI"
    print "Pi equals ".PI."...\n";    # right