It's stated very clearly in File::Glob.

If an error occurred, &File::Glob::GLOB_ERROR will be non-zero and $! will be set. &File::Glob::GLOB_ERROR is guaranteed to be zero if no error occurred

perlvar has more to add on $!. (Emphasis in the original.)

This means that the value of $! is meaningful only immediately after a failure. [...] A successful system or library call does not set the variable to zero.

There's even an example in File::Glob's Synopsis. (The example was adjusted to your case.) 

@list = bsd_glob($pat, GLOB_ERR);

if (GLOB_ERROR) {
   # an error occurred
}

[download]

In reply to Re: bsd_glob does not reset $! by ikegami
in thread bsd_glob does not reset $! by stefan k

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