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Why is it useful to know the type of data a variable holds?

Often the data type of a variable constrains what you can do with it. You may want to make sure that a variable contains an integer or a double before using it in a mathematical calculation, for example.

Should I obey any conventions when naming variables?

Your goal should always be to make your code easy to read and understand. A variable such as $ab123245 tells you nothing about its role in your script and invites typos. Keep your variable names short and descriptive.

A variable named $f is unlikely to mean much to you when you return to your code after a month or so. A variable named $filename, on the other hand, should make more sense.

Should I learn the operator precedence table?

There is no reason that you shouldn't, but I would save the effort for more useful tasks. By using parentheses in your expressions, you can make your code easy to read while defining your own order of precedence.