$ perl -MO=Deparse deb.pl
use Getopt::Long;
BEGIN {${^WARNING_BITS} = "UUUUUUUUUUUU"}
use strict 'refs';
my $FOO_DEBUG_MODE;
sub BEGIN {
    GetOptions 'debug', \$FOO_DEBUG_MODE;
}
use constant ('FOO_DEBUG_MODE', $FOO_DEBUG_MODE);
'???';
print "done!\n";
deb.pl syntax OK