I cannot discern what is eating your memory, but one thing you should do is move your variables declarations into the smallest possible scope. This will remove a lot of cruft from your code which may make it easier to spot the issue.

Eg:

my $shapefile = Geo::ShapeFile->new ("tabblock2010_42_pophu");
my $totalblocks = $shapefile->shapes();
for (1 .. $totalblocks) {
    my %attr = $shapefile->get_dbf_record($_);
    my $blockid = $attr{BLOCKID10};
    ...
[download]

Removing the declarations in the main package, and the statements to clear the variables at the bottom of your loop.

Hope this helps !