adding an appropriate width:nn(units) and max-width:nn (units) to a css declaration for the relevant cells might be a good approach... despite it's non-perlish-ness.

But I can't resist objecting to nested tables.

• They're murder to debug, even when the debugging is in the code, and hellacious to debug in .html
• They (usually) delay rendering (even tho they no longer bring browswers to their knees, as used to be the case)
• ymmv: They're in-elegant unless you are using the tables for a structured, multi-tiered dataset rather than solely for layout

Forgive a disclaimer, please: I'm definitely NOT promoting the notion that "tables (and all other familiar, well-tested, stable html-constructs) are evil; use CSS instead at all expense."

Update: Fixed grammar & typos in last <li...
Add remark: This may reflect a failure of imagination on my part, but wanting a vertical stack of <td>s sounds as though it's inspired by layout concerns.