$ cat deb.pl
#!/usr/bin/perl

use strict;
use warnings;
use Getopt::Long;

my $FOO_DEBUG_MODE;
BEGIN { GetOptions("debug", \$FOO_DEBUG_MODE) }
use constant FOO_DEBUG_MODE => $FOO_DEBUG_MODE;

print "debug statment\n" if FOO_DEBUG_MODE;
print "done!\n";