I'm interested to understand why the '|' math operation decided to return a 3 for me, when given two non-numbers to operate on. This was the fun part.

    (3,4,5) | (7,8,9)
=== ((3,4),5) | ((7,8),9)
=== (4,5) | ((7,8),9)
=== 5 | ((7,8),9)
=== 5 | (8,9)
=== 5 | 9
=== 13

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It's a common misconception that (..., ..., ...) is a list. It's simply two instances of the binary operator ",". In list context, "," returns a list consisting of both operands (which are evaluated in list context). In scalar context, "," returns its RHS operand (after evaluating both sides in scalar context).

Is there a way to write this that will work for both empty and non-empty canvas?

Close. Since you're dereferencing ("@{}") the result of the "||", you need to return an array reference on both sides of the "||". 

my @list2 = @{ $c->bbox('all') || [ 1,2,3 ] };

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Or if you want to avoid creating an array and a reference to it just so it can be derefenced and listed, 

my @list2 = $c->bbox('all');
@list2 = (1,2,3) if !@list2;

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In reply to Re^3: 'or' versus '||' - Unexpected Results by ikegami
in thread 'or' versus '||' - Unexpected Results by cmv

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