$hash{a}{b}{c}='some value';


##</code><code>##

#!/usr/local/bin/perl -w

use strict;

sub set_nested_value {
  my($href, $keys, $value) = @_;
  my $last_key = pop @$keys;

  for my $key (@$keys) {
    $href = $href->{$key} ||= {};
  }

  $href->{$last_key} = $value;
}

my %hash;

set_nested_value(\%hash, ['a' .. 'c'], 'some value');
set_nested_value(\%hash, ['a', 'b', 'd'], 'other value');

use Data::Dumper;

print Dumper \%hash;