debugger output:
...
main::(cheese.pl:4):    my $need_to_interpolate = "smell my %s\n";
  DB<1> s
main::(cheese.pl:5):    printf $need_to_interpolate, $string;
  DB<1> x %s
  empty array
....
[download]

How does the empty hash work, here?

From perldoc -f sprintf:

Perl's "sprintf" permits the following universally-known
conversions:
   %%   a percent sign
   %c   a character with the given number
   %s   a string
   ...

Anguished 'd'uh! /me is shamed, especially since I spent quite while puzzling about the use of printf, and still didn't stumble on the $s being conversion sted some undeclared and empty hash.

Interesting warning there, in how I let -d output shape my train of thought.