Perl Best Practices中一个缺省参数的例子
直接贴代码，忘了时看看，尤其是红色的部分。
1 Readonly my %PAD_DEFAULTS => (
2 cols => 78,
3 centered => 0,
4 filler => $SPACE,
5 # etc.
6 );
7
8 sub padded {
9 my ($text, $arg_ref) = @_;
10
11 # Unpack optional arguments and set defaults...
12 my %arg = ref $arg_ref eq 'HASH' ? (%PAD_DEFAULTS, %{$arg_ref})
13 : %PAD_DEFAULTS;
14
15 # Compute left and right spacings...
16 my $gap = $arg{cols} - length $text;
17 my $left = $arg{centered} ? int($gap/2) : 0;
18 my $right = $gap - $left;
19
20 # Prepend and append space...
21 return $arg{filler} x $left . $text . $arg{filler} x $right;
22 }
