Because of autovivification, $x->{ foo } means ( $x //= { } )->{ foo }.^[1]

As such,

$fs4->{_structure}{'/Users/dotfiles/perl5/my_modules'}{BLAH}{Crap}
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means

(
   (
      (
         (
            $fs4 //= { }
         )->{_structure} //= { }
      )->{ '/Users/dotfiles/perl5/my_modules' } //= { }
   )->{ BLAH } //= { }
)->{ Crap }
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It has nothing to do with exists. (Notice how ->{ Crap } is not created.)

To avoid this, you could use a whole bunch of checks, or you could use the awesome autovivification pragma module.

no autovivification;
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---

1. Some conditions apply.