If it were evaluated right to left, then the answer would negative four

No, it's a per-operator thing, as + and - are different operators, and their precedence defines they should be evaluated left-to-right. This is done by implicit parentheses:

2;0 juerd@ouranos:~$ perl -MO=Deparse,-p -e'sub one { 1 } sub two { 2 
+} sub three { 3 } print one() - three() + two()'
sub one {
    1;
}
sub two {
    2;
}
sub three {
    3;
}
print(((one() - three()) + two()));
[download]

(I had to use these subs, because Perl optimizes 1-2+3 to a literal 0 :)

So we have these calculation:

1. 1 - 3 = -2
2. answer ( = -2 ) + 2 = 0

But which are evaluated first? the 1 or the 3? The -2 or the 2? It becomes clear when you test the operators one by one (or so I thought):

2;0 juerd@ouranos:~$ perl -le'sub foo { print "foo"; return 1 } sub ba
+r { print "bar"; return 2 } print foo + bar'
bar
foo
1
2;0 juerd@ouranos:~$ perl -le'sub foo { print "foo"; return 1 } sub ba
+r { print "bar"; return 2 } print foo && bar'
foo
bar
2
[download]

So I decided to test a lot of operator this way, but found out that everything was evaluated left-to-right, but not if I left out the parens. Why did my first test print bar before foo then? Because I didn't print foo() + bar() but did print foo( +bar() ).

So that still doesn't solve our mistery...

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