"Experimental keys on scalar is now forbidden at -e line 1."

Odd - works fine for me:

$ cat foo
one
two
one
two
three
one
four
$ perl -anE '$h{$F[0]}++}{say qq|$_ appeared $h{$_}| for sort keys %h'
+ foo
four appeared 1
one appeared 3
three appeared 1
two appeared 2
$ perl -v | head -3

This is perl 5, version 26, subversion 1 (v5.26.1) built for x86_64-li
+nux-gnu-thread-multi
(with 62 registered patches, see perl -V for more detail)
$
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Are you sure you substituted single quotes as NetWallah advised?

perl -anE "$h{$F[0]}++}{say qq|$_ appeared $h{$_}| for sort keys %h" *YOUR-FILE*

Unfortunately, this experimental feature no longer exists in perl version 5.24.1 and forward...produces the following error message: "Experimental keys on scalar is now forbidden at -e line 1."

Are you sure? I don't see that feature being used in this script, did you perhaps make a typo when copying it? (Update: Note the $h vs. %h in the final example below.)

$ perl -v
This is perl 5, version 28, subversion 1 (v5.28.1) built for x86_64-li
+nux
...

$ cat foo.txt 
one
two
three
one
five
two
ten
one

$ perl -anE '$h{$F[0]}++}{say qq|$_ appeared $h{$_}| for sort keys %h'
+ foo.txt 
five appeared 1
one appeared 3
ten appeared 1
three appeared 1
two appeared 2

$ perl -anE '$h{$F[0]}++}{say qq|$_ appeared $h{$_}| for sort keys $h'
+ foo.txt 
Experimental keys on scalar is now forbidden at -e line 1.
Type of arg 1 to keys must be hash or array (not scalar dereference) a
+t -e line 1, at EOF
Execution of -e aborted due to compilation errors.
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