Quick and dirty (not recommended):

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

Slightly more respectable:

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

We might be able to help you better if you tell us what you're really trying to do (see XY Problem).