How about:

sub with(\%$) {
   my ($hash, $values) = @_;
   $hash->{$_} = $values->{$_}
      foreach keys %$values;
}

$anonhash = {
   id => 'u1234',
};

with %$anonhash, {
   name    => 'dave',
   city    => 'San Francisco',
   phone   => '555-1212',
   hobbies => [ qw( Perl Anime ) ],
};

require Data::Dumper;
print(Data::Dumper::Dumper($anonhash));

__END__
$VAR1 = {
          'id' => 'u1234',
          'name' => 'dave'
          'city' => 'San Francisco',
          'phone' => '555-1212',
          'hobbies' => [
                         'Perl',
                         'Anime'
                       ],
        };
[download]

With a slight change, you could keep the word "do" (as seen behind the cut), but it's less efficient (because the new values are passed as a list instead of as a hash ref).

sub with(\%%) {
   my ($hash, %values) = @_;
   $hash->{$_} = $values{$_}
      foreach keys %values;
}


$anonhash = {
   id => 'u1234',
};

with %$anonhash, do {
   name    => 'dave',
   city    => 'San Francisco',
   phone   => '555-1212',
   hobbies => [ qw( Perl Anime ) ],
};

require Data::Dumper;
print(Data::Dumper::Dumper($anonhash));

__END__
$VAR1 = {
          'id' => 'u1234',
          'name' => 'dave'
          'city' => 'San Francisco',
          'phone' => '555-1212',
          'hobbies' => [
                         'Perl',
                         'Anime'
                       ],
        };
[download]

Update: Fixed the error pointed out by Tanktalus.