I think what you're asking is "how do I write a postfix if, with an else part?" The answer is a rare one in perl - "you can't".

The following are some alternatives, all equivalent. I have replaced your if(0) with a $test variable which is true 50% of the time.

    use 5.010; #gives you 'say'
    my $test = rand > 0.5;

    # two postfix tests:
    say 'TRUE' if $test;
    say 'FALSE' if not $test;

    # a one-liner conventional if
    if($test) { say 'TRUE' } else { say 'FALSE' }

    # two lines, using 'and' and 'or':
    $test and say 'TRUE';
    $test or say 'FALSE';

    # a ternary (my favourite):
    say $test ? 'TRUE' : 'FALSE';
[download]