# this breaks if @pathparts has a double quote in it!
my $element = '$hash' . join '', map { qq|{"$_"}| } @pathparts;
eval "$element = undef";

##</code><code>##

my $step = \%hash;
while ( my $part = shift $pathparts ) {
    $step->{$part} ||= {};
    $step = $step->{$part};
}