'a.b.c' => 1 # -> { a => { b => {c => 1}}}

cyclic

#!/usr/bin/env perl
use 5.10.0;
use strict;
use warnings;
use utf8;
 
use Test::More;
 
sub merge {
    my ($a, $b) = @_;
 
    while (my ($k, $v) = each %$b) {
        if (ref $v) {
            $a->{$k} //= +{};
            merge($a->{$k}, $v);
        }
        else {
            $a->{$k} = $v;
        }
    };
    $a;
}
 
 
sub _hoge {
    @_ == 1 ? shift : +{ shift() => _hoge(@_) }
}
 
sub hoge {
    my $h = shift;
 
    my $res = +{};
    while (my ($key,$val) = (each %$h)) {
        $res = merge($res, _hoge( (split /\./, $key), $val));
    }
    $res;
}
 
 
my $ORG = +{
    'a.b.c' => 1,
    'a.b.d' => 'hello',
    'e.f'   => 'world',
    'g'     => 'apple',
    #'a'     => 'aaa', # ぬーん
};
 
is_deeply hoge($ORG), +{
    a => +{
        b => +{
            c => 1,
            d => 'hello',
        },
    },
    e => +{
        f => 'world',
    },
    g => 'apple',
};
 
done_testing;

gistfile1.pl

use strict;
use warnings;
use utf8;
 
use Test::More;
 
sub merge {
    my ($a, $b) = @_;
 
    while (my ($k, $v) = each %$b) {
        if (ref $v) {
            $a->{$k} //= +{};
            merge($a->{$k}, $v);
        }
        else {
            $a->{$k} = $v;
        }
    };
    $a;
}
 
 
sub hoge {
    my $h = shift;
 
    my $res = +{};
    while (my ($key,$val) = (each %$h)) {
 
        my $sub;
        my @keys = split /\./, $key;
        while (my $k = pop @keys) {
            $sub = +{ $k => ($sub ? $sub : $val) };
        }
        $res = merge($res, $sub);
    }
    $res;
}
 
 
my $ORG = +{
    'a.b.c' => 1,
    'a.b.d' => 'hello',
    'e.f'   => 'world',
    'g'     => 'apple',
    #'a'     => 'aaa', # ぬーん
};
 
is_deeply hoge($ORG), +{
    a => +{
        b => +{
            c => 1,
            d => 'hello',
        },
    },
    e => +{
        f => 'world',
    },
    g => 'apple',
};
 
done_testing;