When does (pre|post)increment happen?

Answer

Perl evaluation order is as follows

• Depth-first
• Left-to-right
• Side-effects happen immediately
• lvalued expressions in an rvalue context converted as late as possible
• ++$n is lvalued despite what the the compiler says

Do not rely on all this staying the same in future versions

$n = 1;
print $n / ++$n;

• LHS operand of / evaluated to lvalue $n
• RHS operand of / is evaluated to lvalue $n, incrementing $n to 2 at the same time
• lvalues $n,$n converted to rvalues 2,2
• 2/2 evaluated to 1

$n = 1;
print $n / $n++;

• LHS operand of / evaluated to lvalue $n
• RHS operand of / is evaluated to rvalue 1, incrementing $n to 2 at the same time
• lvalue $n converted to rvalue 2
• 2/1 evaluated to 2

Brian McCauley's original answer