To answer your first question, you could always check to see if the symbol table entries exist. For example:

> perl
print "Data::Dumper exists\n" if %Data::Dumper::;
> perl -MData::Dumper
print "Data::Dumper exists\n" if %Data::Dumper::;
Data::Dumper exists
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So you could do:

if (%HTML::Template::) {
  # HTML::template stuff
} else {
  # Regular HTML stuff
}
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Keep in mind, however, there is a caveat to that: if you load HTML::Template::othermodule without loading HTML::Template, that may fail. Instead, you can do:

if (exists ${%HTML::Template::}{'import'}) {
  ...
}
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which checks for the existance of the import subroutine. import is in every package by default, and that can be used to see if it exists (and hence if a module has been loaded by that name).