($newValue) = $oldValue =~ /_(\d+)\./;

##</code><code>##

($newValue) = $oldValue =~ /_(\d+)\./ or warn "bad format: '$oldValue'";

##</code><code>##

if (($newValue) = $oldValue =~ /_(\d+)\./) {
   # all is ok
} else {
   # something went wrong
}