What exactly do you expect from broken code?

$ perl -E' $_[0] = {}; say keys %$_[0]'
Experimental keys on scalar is now forbidden at -e line 1.
Type of arg 1 to keys must be hash or array (not index/value array sli
+ce) at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
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Please note the not index/value array slice , because that's what's fed into keys.

Try keys %{$_}[0] and compare the error messages...

Update

Admittedly, I've never used this before: Index/Value-Array-Slices