...
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";

...

##</code><code>##

use constant READ_PERM => READ_PERM
use constant WRITE_PERM => WRITE_PERM;
use constant EXECUTE_PERM => EXECUTE_PERM;

my @allowed_perms = (
    READ_PERM,
    EXECUTE_PERM
);

# Check permissions
if ( scalar(@allowed_perms) == 0 )
{
    print "PERMISSION DENIED\n";
    exit -1;
}

if ( grep { $_ == READ_PERM } @allowed_perms )
    print "READ Allowed\n";

if ( grep { $_ == WRITE_PERM } @allowed_perms )
    print "WRITE Allowed\n";

if ( grep { $_ == EXECUTE_PERM } @allowed_perms )
    print "EXECUTE Allowed\n";