$ perl -MO=Deparse
use constant DOFOO => 0;
my $DOBAR = 0;
print "foo" if  DOFOO;
print "bar" if $DOBAR;
__END__
use constant ('DOFOO', 0);
my $DOBAR = 0;
'???';
print 'bar' if $DOBAR;

##</code><code>##

use Data::Dump;
use constant FOO => 123;
my $x = 455;
sub BAR { ++$x }
my %h = (
         FOO  => "A",
         BAR  => "B",
        (FOO) => "C",
        (BAR) => "D",
);
dd \%h;
__END__
{ 123 => "C", 456 => "D", BAR => "B", FOO => "A" }