Without any surrounding context it's hard to determine what you're trying to accomplish here. Consider that $x .= $y is just shorthand for $x = $x . $y. So what you've written is this:

$month_3_tables . $x = $month_3_tables . $x . "html stuff"
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which isn't meaningful, as you need to have a single variable name on the left side of the assignment statement.

Is your intent to modify individual elements within an array, like this?

$month_3_tables[$x] .= "html stuff"
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It might help if you followed up with a bigger snippet of code and/or a more complete explanation of what you expected the behavior to be.

        $perlmonks{seattlejohn} = 'John Clyman';