Turns out IPC::System::Simple ISN'T a core modure, even though it's used by core module autodie. use autodie qw( system ); fails if IPC::System::Simple isn't installed.

$ perl -e'use autodie qw( system );'
IPC::System::Simple required for Fatalised/autodying system() at -e li
+ne 1
BEGIN failed--compilation aborted at -e line 1.
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That's why use autodie; aka use autodie qw( :default ); doesn't include use autodie qw( system );.

That's a huge wtf, especially considering the usefulness of what the module provides: It provides alternatives to questionably-designed builtin functions.