use strict;
use constant (STR => 't', FLAG_X => 1);

if (FLAG_X && test()) {
print "OK";
}

sub test {
return 1;
}
Bareword "FLAG_X" not allowed while "strict subs" in use at t.pl line 4.
Execution of t.pl aborted due to compilation errors.

これはなぜなのでしょうか?
複数の定数を定義するとこうなるのですが