Comment by interroboink

The footnote is originally referenced here:

    Figure 6 represents the string “gggijgziifiiffif”, which by pure
    coincidence happens to accurately represent the authors’ verbal reaction
    upon learning that “unquoted strings” were a feature intentionally
    included in the Perl language.⁵

So, the hint is that this has to do with the "unquoted strings" feature (aka "bare words"[1]).

See the sibling comment about the actual parse — "at" and "tmp" are seen as strings.

The strings get coerced into numbers due to being used with the numeric "/" operator (that's normal Perl behavor). Since the strings can't be parsed as numbers, they become "0". So, you get division by 0.

[1] https://perlmaven.com/barewords-in-perl