How many spaces to use for each indentation level is actually something that has been academically studied. The answer is "3 or 4 spaces". 2 is too few for good readability and 5 winds up taking up more space while not improving readability. 3 or 4 appear to be almost the same. Certainly 1 is too few. The human eye will just get lost.

To use tabs or not in the code is something that can start a long, very emotional discussion. I personally do not put tabs anywhere in the code or comments.

If you use an editor that is designed to be used for writing code, there will be special features that make it easy to enforce whatever style you prefer. For example, my editor has an option, "convert tabs to spaces". Without doing something special, I can't wind up with any embedded tab characters.

There is a fair amount of variability on the "braces style". One way is like you did it. For Perl code, I prefer to put the initial opening brace on its own line. Like this:

for ($a=0; $a<=2; $a++)
{
    for ($b=0; $b<=2; $b++)
    {
        $m_avrg[$a][$b] = ($m_avrg[$a][$b] + $list[$a][$b]);
        print  "$m_avrg[$a][$b] \n";
    }
}
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For other languages like Java, I use the more vertically compact form because there winds up being a whole mess of little itty bitty "getters and setters". So I am flexible about this point, depending upon the situation.