You obviously didn't see this bit
The support that exists for externals definitions in Subversion today can be a little misleading, though. First, an externals definition can only point to directories, not files.
Nor have you ever tried to external a single file, which will choke with a message about only working on directories. There's also 3years of development discrussion on svn-devel about this functionality, and several unresolved tickets where the potential for support wavers back and forth.

In reply to Re^2: svn externals and perl packages by nmerriweather
in thread svn externals and perl packages by nmerriweather

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