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

$ 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) at -e line 1, at EOF
Execution of -e aborted due to compilation errors.