t/test1.t
t/test1_a.t

##</code><code>##

system "perl t/test1.t -a qqq -b 1";

##</code><code>##

BEGIN {

        $error = 0;

        # foo:bar is required
        eval {
                require foo::bar;
                };
        if ( $@ ) {
                warn "foo::bar is not installed or is installed incorrectly - skipping test1_a.t";
                $error = 1;
                }
        }

exit(0) if ( $error == 1 );
system "perl t/test1.t -a qqq -b 1";

##</code><code>##

t/test1_a......foo::bar is not installed or is installed incorrectly - skipping test1_a.t at t/test1_a.t line 13.
FAILED before any test output arrived

##</code><code>##

t/test1_a........................foo::bar is not installed or is installed incorrectly - skipping test1_a.t