Some small things I found in the text:

The document provides two implementation
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The document provides two implementations

have the same preceedence
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have the same precedence

The following subsections show how to evaluate the list how to treeify it.
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The following subsections show how to evaluate the list and how to treeify it.

In a list, that means the means the last (right-most) element encountered will be executed first.
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In a list, that means the last (right-most) element encountered will be executed first.

Fortuantely, Parse::RecDescent provides a means
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Fortunately, Parse::RecDescent provides a means

and each reference to that rule
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and of each reference to that rule

Just like function can have arguments, so can subrules.
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Just like functions can have arguments, so can subrules.

However, Parse::RecDescent handles rule with productions
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However, Parse::RecDescent handles rules with productions

Just like Parse::RecDescent has an directive for creating a flat list for a left-associative operator (<lefttop>),
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Just like Parse::RecDescent has a directive for creating a flat list for a left-associative operator (<leftop>),

The problem we have to pass $item[1] from pow to pow_.
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The problem is we have to pass $item[1] from pow to pow_.

The following subsections contains complete, working code
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The following subsections contain complete, working code