$i++ is the equivalent of

$i=$i+1;
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where the = is an operator. It's not just sitting there, like it does in math, saying "these two things are equal"; it tells Perl to make the two things equal. So here it's saying "take the current value of $i and add 1, and make that the new value of $i." Then the for loop continues with the new value of $i. This happens each time the loop runs, until $i reaches 100.

_________
"Abby-somebody. Abby-normal."
—Young Frankenstein