gistfile1.pl

#!perl

use strict;
use warnings;
use Data::Dumper;

my $ref = {
    foo => {
        qux => {
            quux => 2,
        },
        bar => {
            baz => 1,
        },
    },
};

_set_value( $ref, 'foo.qux.quux', 3 );
_set_value( $ref, 'foo.qux.quux.bar', 3 );
warn Dumper $ref;

sub _set_value {
    my ( $a_ref, $key, $value ) = @_;
    my $p = $a_ref;
    for my $path ( split /\./, $key ) {
        if ( ref $p->{ $path } && $key !~ /$path$/ ) {
            $p = $p->{ $path };
        }
        else {  
            if ( $key =~ m/$path$/ ) {
                $p->{ $path } = $value;
                last;
            }
            else {
                $p->{ $path } = {};
                $p = $p->{ $path };
            }
        }

    }
}