You could make your "success" return value(s) dual-type (see dualvar) with the numeric part being zero, in which case you could simply test if the value is less than zero:

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";    # <--
        }
}
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The main purpose of this is that the code would run cleanly under strictures, i.e. without eliciting the warning 'Argument "true\n" isn't numeric in numeric lt (<)'.

In other words, your original regular scalars would in principle work, too (a string which doesn't look like a number, evaluates to zero), but you'd have to say no warnings 'numeric'; — which ultimately could get even more unwieldy than what you have now, in case you try to restrict it to the minimal scope required every time...