...
use constant READ_PERM => 0x04;
use constant WRITE_PERM => 0x02;
use constant EXECUTE_PERM => 0x01;
use constant NO_PERM => 0x00;

my $allowed_perms = READ_PERM | WRITE_PERM;

# Check for permissions:
if ( $allowd_perms == NO_PERM )
{
    print "PERMISSION DENIED\n"
    exit -1;
}

if ( $allowed_perms & READ_PERM )
    print "READ Allowed\n";

if ( $allowed_perms & WRITE_PERM )
    print "WRITE Allowed";

if ( $allowed_perms & EXECUTE_PERM )
    print "EXECUTE Allowd\n";

...