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$ perl -e'my $x = { }; keys $x;'
Experimental keys on scalar is now forbidden at -e line 1.
Type of arg 1 to keys must be hash or array (not private variable) at 
+-e line 1, at EOF
Execution of -e aborted due to compilation errors.

$ perl -e'use feature qw( state ); ( state $x ) = 123;'
Initialization of state variables in list currently forbidden at -e li
+ne 1, near "123;"
Execution of -e aborted due to compilation errors.

$ perl -e'use feature qw( state ); state( $x ) = 123;'
Initialization of state variables in list currently forbidden at -e li
+ne 1, near "123;"
Execution of -e aborted due to compilation errors.

$ perl -e'$/ = \0;'
Setting $/ to a reference to zero is forbidden at -e line 1.

$ perl -e'$/ = { };'
Setting $/ to a HASH reference is forbidden at -e line 1.

$ perl
my $x = <<.
Use of bare << to mean <<"" is forbidden at - line 1.

$ perl -e'my $x = chr( 0x2660 ); vec( $x, 0, 1 );'
Use of strings with code points over 0xFF as arguments to vec is forbi
+dden at -e line 1.