If you do not want to modify the value of $middle (so you leave it as undef, and not an empty string), you may want to use printf:

printf "last:(%s)  first(%s) middle(%s)\n",
  $last || '', $first || '', $middle || '';
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or if you've got a whole lot to deal with

printf "some format string goes here",
  map { defined($_) ? $_ : '' } (@list_of_variables);
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Or, there's always:

{ no warnings 'uninitialized';
  print "whatever";
}
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