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

Initially, you might gather the data using a simple loop:

my %provisions;
my $person;
while (<>) {
  if (/^(\S.*)/) { # a person's name (no leading whitespace)
    $person = $1;
    $provisions{$person} = [  ] unless exists $provisions{$person};
  } elsif (/^\s+(\S.*)/) { # a provision
    die "No person yet!" unless defined $person;
    push @{ $provisions{$person} }, $1;
  } else {
    die "I don't understand: $_";
  }
}

First, you declare the variables for the resulting hash of provisions and the current person. For each line that is read, determine if it's a person or a provision. If it's a person, remember the name and create the hash element for that person. The unless exists test ensures that you won't delete someone's provision list if his list is split in two places in the data file. For example, suppose that "The Skipper" and " sextant" (note the leading whitespace) are at the end of the data file in order to list an additional data item.

The key is the person's name, and the value is initially a reference to an empty anonymous array. If the line is a provision, push it to the end of the correct array, using the array reference.

my %provisions;
my $person;
while (<>) {
  if (/^(\S.*)/) { # a person's name (no leading whitespace)
    $person = $1;
    ## $provisions{$person} = [  ] unless exists $provisions{$person};
  } elsif (/^\s+(\S.*)/) { # a provision
    die "No person yet!" unless defined $person;
    push @{ $provisions{$person} }, $1;
  } else {
    die "I don't understand: $_";
  }
}

What happens when you try to store that blue shirt for the Skipper? While looking at the second line of input, you'll end up with this effect:

push @{ $provisions{"The Skipper"} }, "blue_shirt";

At this point, $provisions{"The Skipper"} doesn't exist, but you're trying to use it as an array reference. To resolve the situation, Perl automatically inserts a reference to a new empty anonymous array into the variable and continues the operation. In this case, the reference to the newly created empty array is dereferenced, and you push the blue shirt to the provisions list.

This process is called autovivification. Any nonexisting variable, or a variable containing undef, which is dereferenced while looking for a variable location (technically called an lvalue context), is automatically stuffed with the appropriate reference to an empty item, and the operation is allowed to proceed.

For example, this works:

my $not_yet;                # new undefined variable
@$not_yet = (1, 2, 3);

Here, you dereference the value $not_yet as if it were an array reference. But since it's initially undef, Perl acts as if you had said:

my $not_yet;
$not_yet = [  ]; # inserted through autovivification
@$not_yet = (1, 2, 3);

In other words, an initially empty array becomes an array of three elements.

This autovivification also works for multiple levels of assignment:

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