Do I actually have to have multiple templates?

No, you can include multiple templates within a single file using

  <TMPL_IF template1>...</TMPL_IF>
  <TMPL_IF template2>...</TMPL_IF>
  etc.
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but it isn't generally advisable. I keep mine separate template files.

I guess what I'm trying to say is that I don't see the benefit of using HTML::Template if I have X number of templates, where each has to be edited manually later.

Some might call that an advantage. If you have a lot of repetive code, <TMPL_INCLUDE> can ease the pain.