Here's the original mess, which usually worked but sometimes did not, and looks messy: fragile, unmaintainable. A block of code is set up for string eval:

    if ( not $code ) {
        $code       = $unit;
    };
    if ( $args and ref $args ) {
        @args       = @$args;
        $code       .= q{(@args)};
    }
    else {
        @args       = ();
    };
    if ( $argv and ref $argv ) {
        @ARGV       = @$argv;
    }
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Both $code or $args may be supplied or not; and an attempt is made to exhaust all four possible cases. As you can see, I've painted myself into such a corner that I've introduced a third input variable.

Another approach, which truly annoys me, is to nest conditionals:

    if ( $A ) {
        if ( $B ) {
            {...}
        }
        else {
            {...}
        };
    }
    else {
        if ( $B ) {
            {...}
        }
        else {
            {...}
        };
    };
[download]

This is so ugly and miserable I've rarely employed it. The second evaluation of the nested comparison, whatever it may be, is particularly offensive.

Alternatively, exhaust all four cases explicitly, at the cost of introducing two scratch flags:

• Sets out all four cases explicitly.
• All four cases at same level.
• Flags guarantee simple boolean state.
• Eliminates the wacky third input variable.