sub setpermbybitnum { my $vec=""; vec($vec,shift,1) = 1; $vec }

# check if any perms are set
sub no_perm { "$_[0]" =~ tr/\0//c == 0}

# check if all specified perm(s) are set
sub has_perms { no_perm( ("$_[0]" & "$_[1]") ^ "$_[1]") }

# check if any specified perms are set
sub one_or_more_of_perms { !no_perm("$_[0]" & "$_[1]") }

use constant READ_PERM => setpermbybitnum(3);
use constant WRITE_PERM => setpermbybitnum(2);
use constant EXECUTE_PERM => setpermbybitnum(1);

my $allowed = READ_PERM | WRITE_PERM;

# Check for permissions:

if ( no_perm($allowed) )
{
    print "PERMISSION DENIED\n";
    exit -1;
}

if ( has_perms($allowed, READ_PERM) ) {
    print "READ Allowed\n";
}

if ( has_perms($allowed, WRITE_PERM) ) {
    print "WRITE Allowed\n";
}

if ( has_perms($allowed, EXECUTE_PERM) ) {
    print "EXECUTE Allowd\n";
}

if ( has_perms($allowed, READ_PERM|WRITE_PERM ) ) {
    print "READ and WRITE Allowed\n";
}

if ( has_perms($allowed, READ_PERM|EXECUTE_PERM) ) {
    print "READ and EXECUTE Allowed\n";
}

if ( one_or_more_of_perms($allowed, READ_PERM|EXECUTE_PERM) ) {
    print "READ or EXECUTE Allowed\n";
}