That's only part of the story.

Even though I'm no C expert, I did some hunting around in the internals of File::Glob. bsd_glob calls doglob. In addition, the module uses XSLoader, so, it's an XS module. :) Looking at the current source of Glob.xs, doglob in turn calls the C function (not an XS function) bsdglob, which in turn can be found in bsd_glob.c. In turn, that calls the C function glob0, in the same source file. Are you still with me? Now the documentation for glob0 says:

/*
 * The main glob() routine: compiles the pattern (optionally processin
+g
 * quotes), calls glob1() to do the real pattern matching, and finally
 * sorts the list (unless unsorted operation is requested).  Returns 0
 * if things went well, nonzero if errors occurred.  It is not an erro
+r
 * to find no matches.
 */
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So, it appears to be the "main routine".

Now, and this is the interesting part: it allocates buffers with a length MAXPATHLEN. This constant is defined in perl.h, and it's platform dependent. Anyway, usually this constant is an IMO rather small number, as the code comments there state:

#       define MAXPATHLEN 1024    /* Err on the large side. */
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Note the comment: 1024 is considered "large".

Now, to answer the OP's question: yes, it looks like there's a built in limit on the length of the strings that glob will return, but the exact maximum value depends on your platform.

And I'm not sure I did all of my homework well, I might still have overlooked something essential.