#! perl -slw
use strict;
use Hash::Util qw[ lock_hash ];
use constant FOO => { x => 0 };
lock_hash( %{ FOO() } );

++ FOO->{x};

print FOO->{x};
__END__
C:\test>junk7
Modification of a read-only value attempted at C:\test\junk7.pl line 7.