Since the first expression glob("$_/f*") is only true for 'dir1', it should only return that element of the directory list (even though glob returns 3 files in that directory).

You are missing the fact that grep puts the glob in a scalar context, and that makes it act as an iterator:

In scalar context, glob iterates through such filename expansions, returning undef when the list is exhausted.
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In reply to Re^3: Using grep and glob to find directories containing file by BrowserUk
in thread Using grep and glob to find directories containing file by Anonymous Monk

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