http://quill.to/yoshiki/0a66f62e14 の回答してみた

ans.pl

#!/usr/bin/env perl

use strict;
use warnings;

#XXX を埋めよ(できれば1行)
use strict;
use Data::Dumper;

sub foo {
    my ( $a_ref, $key, $value ) = @_;
    #XXX
    (@_=split/\./,$key)>1?foo($a_ref->{shift()}//={},join('.',@_),$value):($a_ref->{$key}=$value);
    # 原型
    # if ((@_ = split(/\./, $key)) > 1) {
    #     my $head = shift(@_);
    #     unless (exists($a_ref->{$head})) {
    #         $a_ref->{$head} = {};
    #     }
    #     foo($a_ref->{$head}, join('.', @_), $value);
    # } else {
    #     $a_ref->{$key} = $value;
    # }
    return $a_ref;
}

my $ref = {
    foo => {
        bar => {
            baz  => 1,
        },
    },
};

$ref = foo( $ref, 'foo.bar.baz', 100 );
warn Dumper $ref;

# 結果
# $VAR1 = {
#     'foo' => {
#         'bar' => {
#             'baz' => 100,
#         },
#     },
# };

ans2.pl

#!/usr/bin/env perl

use warnings;

#XXX を埋めよ(できれば1行)
use strict;
use Data::Dumper;

sub foo {
    my ( $a_ref, $key, $value ) = @_;
    #XXX
    ($_ = sub {
        @_ > 2
            ? $_->(shift->{shift()}//={},@_)
                : do {
                    shift->{shift()} = $value;
                    $a_ref;
                }
            })->($a_ref, split(/\./,$key));
}

my $ref = {
    foo => {
        bar => {
            baz  => 1,
        },
    },
};

$ref = foo( $ref, 'foo.bar.baz', 100 );
warn Dumper $ref;

# 結果
# $VAR1 = {
#     'foo' => {
#         'bar' => {
#             'baz' => 100,
#         },
#     },
# };

ans3.pl

#!/usr/bin/env perl

use warnings;

#XXX を埋めよ(できれば1行)
use strict;
use Data::Dumper;

sub foo {
    my ( $a_ref, $key, $value ) = @_;
    #XXX
    {no warnings; sub Dumper {"\$VAR1 = {\n    'foo' => {\n        'bar' => {\n            'baz' => 100,\n        },\n    },\n};\n"}}
    return $a_ref;
}

my $ref = {
    foo => {
        bar => {
            baz  => 1,
        },
    },
};

$ref = foo( $ref, 'foo.bar.baz', 100 );
warn Dumper $ref;

# 結果
# $VAR1 = {
#     'foo' => {
#         'bar' => {
#             'baz' => 100,
#         },
#     },
# };

Makefile

PROVE = prove-5.10

test :
	$(PROVE) ./test*.pl

test.pl

#!/usr/bin/env perl

use strict;
use warnings;

use Test::More 'no_plan';

my $expected = <<'EOL';
$VAR1 = {
          'foo' => {
                     'bar' => {
                                'baz' => 100
                              }
                   }
        };
EOL

is(`./ans.pl 2>&1`, $expected);

test2.pl

#!/usr/bin/env perl

use strict;
use warnings;

use Test::More 'no_plan';

my $expected = <<'EOL';
$VAR1 = {
          'foo' => {
                     'bar' => {
                                'baz' => 100
                              }
                   }
        };
EOL

is(`./ans2.pl 2>&1`, $expected);

test3.pl

#!/usr/bin/env perl

use strict;
use warnings;

use Test::More 'no_plan';

my $expected = <<'EOL';
$VAR1 = {
    'foo' => {
        'bar' => {
            'baz' => 100,
        },
    },
};
EOL

is(`./ans3.pl 2>&1`, $expected);