How can I solve this?

Also use Geography::NationalGrid, i.e.

use  Geography::NationalGrid;
use  Geography::NationalGrid::IE;
...
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(This is required for the constant declarations such as

use constant MIN_LONG => Geography::NationalGrid->deg2rad('-11d 12m 0s
+');
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in Geography::NationalGrid::IE. — The module should probably load Geography::NationalGrid itself (as it obviously depends on it), but it doesn't...)