Essentially, any time you are operating in a block (typically defined by statements that use { and } to denote a block), you indent a single time:

for (@things){
    # this is a block. indent once

    if ($_ eq 'blah'){
        # this is an inner block, indent twice

        while (1){
            # 2nd level inner block
            # indent three times
        }

        if (0){
            # another 2nd level inner block
        }

        # back in 1st inner block
    }

    # back in outer block
}

# back to top level
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