gistfile1.PL

use strict;
use Data::Dumper;

sub foo {
    my ( $a_ref, $key, $value ) = @_;
    my $a_ref2 = \$a_ref;
    $a_ref2 = \$$a_ref2->{$_} for split /\./, $key;
    $$a_ref2 = $value;
    return $a_ref;
}

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

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