perl -MDevel::Peek -e 'Dump(%{$_[0]})'
SV = PVHV(0x157010280) at 0x15700ae40
  REFCNT = 1
  FLAGS = (SHAREKEYS)
  ARRAY = 0x0
  KEYS = 0
  FILL = 0
  MAX = 7

But what is this:

perl -MDevel::Peek -e 'Dump(%$_[0])'  
SV = NULL(0x0) at 0x100939eb8
  REFCNT = 2147483641
  FLAGS = (READONLY,PROTECT)

and this:

perl -MDevel::Peek -e 'Dump(%$^[0])'  
SV = IV(0x159825cf0) at 0x159825d00
  REFCNT = 1
  FLAGS = (PADTMP,IOK,pIOK)
  IV = 5796572328

perl -MDevel::Peek -e 'Dump(%{$^[0]})'
Number found where operator expected (Missing operator before "0"?)
[download]

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.
[download]

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

In other words an array slice of a hash ... WT(sic)F?

Update

Without keys the error is clearer %$X[0] is a less common dereferencing syntax, an alternative notation to $X->{0} but here it's an array index [0].

That's nonsense, but the fact that keys' error message is kicking in earlier is masking the issue.

Additionally is $_ undef and leaving room for auto vivification.

Quite a mess man...

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}