array in hash value

#!/usr/bin/perl -w
#
use strict;

my %provisions;
my $person;

while(<>){
    if(  /^(\S.*)/){
        $person = $1;
        $provisions{$person} = [ ]; #this can be omitted.
    }elsif(/^\s+(\S.*)/){
        push @{$provisions{$person}}, $1;
    }else{
        die "check $_";
    }

}

for( keys %provisions){
    print "$_\n";
    my $r = $provisions{$_};
    print "@$r, \n";
}

the above code will put the containt like：

The Skipper
  blue_shirt
  hat
  jacket
  preserver
  sunscreen
Professor
  sunscreen
  water_bottle
  slide_rule
Gilligan
  red_shirt
  hat
  lucky_socks
  water_bottle

in hash. if the value of a hash is array, we cannot put the array directly to the value but give the reference of the array.

上述code中，可以把 $provisions{$person} = [ ];删除。当调用
push @{ $provisions{'The Skipper'} }, "blue_shirt"; 时，$provisions{"The Skipper"} 是不存在的。这时perl会自动的插入一个reference赋给它。这个reference指向一个匿名空数组。然后再继续push动作。This process is called autovivification.

Any nonexistent variable, or a variable containing undef, which we dereference while looking for a variable location (technically called an lvalue context), is automatically stuffed with the appropriate reference to an empty item, and Perl allows the operation to proceed.

my $top;
$top->[2]->[4] = 'lee-lou';

最初$top是undef,因为我们像用一个引用一样来解引用$top。所以perl把一个匿名空数组的引用赋给$top.接下来perl去访问第三个元素，这让数组的长度变长为三。这个元素还是个undef，所以perl又赋一个匿名空数组的引用给第三个元素。最后把lee-lou赋给第二次解引用的第五个元素。