I've downloaded File::Sync from CPAN. I found the below line of code in Sync.pm and I don't understand, how this function is working

# Interface from Perl filehandle to POSIX file descriptor.
sub fsync(*) {
    @_ == 1 or croak "usage: fsync FILEHANDLE";

    fsync_fd(fileno(qualify_to_ref($_[0], caller())));
}

# Make fsync a method of IO::Handle and FileHandle.
*IO::Handle::fsync = *FileHandle::fsync = \&fsync;
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I don't know where the fsync_fd function is defined? Can someone clarify me.