or download this
$ perl -wle'use re "debug"; $x = "foo\nbar"; qr/(?x)hullo (?#there $x)
+/'
Compiling REx `(?x)hullo (?#there $x)'
...
5[6] 0[0] 0[0] 23[0]
Name "main::x" used only once: possible typo at -e line 1.
Freeing REx: `"(?x)hullo (?#there $x)"'