perl -e "@m=glob('*');@m = grep{m/\d+_\d+\.REPT/g} @m; foreach(@m){$s=$_;$s=~s/REPT/ERR/; rename($_,$s);}" 

##</code><code>##

perl -e "foreach(grep {m/(\d+_\d+)\.REPT/g} glob('*')){$aa=$_; rename($aa,$_) if $_=~s/REPT/ERR/;};"