I don't think that one is a mistake. At least, if it's a mistake, it's not a mistake in the equals operator. PHP attempted to use one set of comparison operators and the fallout has been far worse: there have been security problems due to the semantics of PHP's == operator attempting to guess whether to perform string or numeric comparison when doing password comparisons.

The mistake, if any, is that numbers and strings are unified into one scalar type in Perl. This allows some tricks, such as being able to use sprintf to perform rounding, but it has fallout in other places in the language, such as here. Given that numbers and strings are unified, having two operators for comparison is the correct decision: it makes the semantics much simpler by having the language not guess which kind of comparison to perform.

They're not mistakes; read the article. "Equality" is not the singular, unambiguous concept that many programmers think it is.