I don't know what the previous anonymonk talks about, but Geo::ShapeFile caches data, which probably eats a lot of memory with your 421545 records. Try $shapefile->caching('dbf', 0) (appears to be undocumented). Also, what 1nickt said.

In reply to Re: Geo::ShapeFile memory problem by Anonymous Monk
in thread Geo::ShapeFile memory problem by rhzhang

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