This looks like a snippet from a make file.

This:

@folder_mk =./AACBaseFileLib/common.mk
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...is the target. That is, someone would type make @folder_mk to run this section of the make file.

This:

$(PROJECT_ROOT)/../FilemuxIn
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...is dereferencing a make variable in the prerequistes section.

The rest is perl code (probably) that seems to be in the recipe section. This is the code that gets run in order to build the @folder_mk target.

You are going to have to understand make. Here is the link to docs: http://www.gnu.org/software/make/manual/make.html. Once you figure out the preprocessing that make is doing, then you can tell us the full code that is getting executed.