IO::Handle's sync requires an output file handle. I don't know if Perl provides a means of getting an output file handle to a directory. Otherwise, it's just a wrapper for C's fsync.

Update: You can get one via dup:

$ perl -MIO::Handle -e'
   open(my $fh, "<", $ARGV[0]) or die;
   open(my $ofh, ">&", $fh) or die;
   $ofh->sync or die;
   print "success\n"
' some_dir
success
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