use strict;
use warnings;
use Scalar::Util 'dualvar';

for(my $ii = 1; $ii < 4; $ii++)
{
        my $a = foo($ii);
        if($a < 0)         # <--
        {
                print "Fail occured with code: $a\n";
        }
        else
        {
                print "Success\n";
        }
}

sub foo
{
        my $num = shift;

        if($num == 1)
        {
                return -1;
        }
        elsif($num == 2)
        {
                return -2;
        }
        else
        {
                return dualvar 0, "true\n";    # <--
        }
}