I noticed that "make install" on Linux installs a manual page for ExtUtils::XSSymSet (e.g. /usr/local/share/man/man3/ExtUtils::XSSymSet.3), but the corresponding Perl module is missing (ExtUtils/XSSymSet.pm).

The explanation is in ./installperl:

sub installlib {
    ...
    return if $name eq 'ExtUtils/XSSymSet.pm' and !$Is_VMS;
    ...
}

It seems the same exception is not applied on manual pages.

Another possible fix would be removing the exception and installing the module unconditionally since the module can be used as a general string shortener:

"ExtUtils::XSSymSet" provides functions to
shorten names in a consistent fashion, and to track a set of names to
insure that each is unique. While designed with xsubpp in mind, it may
be used with any set of strings.