$ cat connie.pl
use constant PI    => 4 * atan2(1, 1); use constant DEBUG => 0; 1;

$ perl -e"require q!connie.pl!; warn PI;
PI at -e line 1.

$ perl -e"require q!connie.pl!; warn PI();
3.14159265358979 at -e line 1.

$ perl -e"BEGIN{ require q!connie.pl!;} warn PI;
3.14159265358979 at -e line 1.