I suspect a subtlety (bug?) in Text::ParseWords or its usage.

The documentation of this module is quite explicit about how it works:

The $keep argument is a boolean flag. If true, then the tokens are split on the specified delimiter, but all other characters (quotes, backslashes, etc.) are kept in the tokens. If $keep is false then the &*quotewords() functions remove all quotes and backslashes that are not themselves backslash-escaped or inside of single quotes (i.e., "ewords() tries to interpret these characters just like the Bourne shell).

The *quotewords functions all call parse_line, which is the one that performs the real job. File::Glob calls parse_line with a $keep argument of 0:

if ($pat =~ /\s/) {
    # XXX this is needed for compatibility with the csh
    # implementation in Perl.  Need to support a flag
    # to disable this behavior.
    require Text::ParseWords;
    @pat = Text::ParseWords::parse_line('\s+',0,$pat);
}
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So, knowing this, it comes as no surprise that some backslashes are being eaten. As soon as I replace the 0 with a 1, the behaviour of glob begins to match my expectations. What bothers me is that File::Glob is one of that pieces of software so widely used that it's impossible that this little humble programmer have found a bug in it :^).