Answering my own question:

Image::Magick allows to crop rows and columns, trimming the result can show if they are "empty".

 
$x = $p->Crop(geometry=>'439x4+166+167');
warn "$x" if "$x";
$x = $p->Trim();
warn "$x" if "$x";

print join ",",$p->Get(qw/columns rows page/);
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output

Exception 310: geometry does not contain image `216.PNG' @ warning/att
+ribute.c/GetImageBoundingBox/239 at /tmp/tst_im.pl line 36.
1,1,1238x1688-1-1
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so a clever combination of crops and trims can quite effectively dissect a table into it's cells.

Of course one needs reasonable assumptions about the geometry.