Thank you for the explanation, which was very clear and made perfect sense. I think I now understand what is going on. Is variable interpolation suppressed inside a (?#...) construct? That would explain why it stopped the warning. Let's see

$ perl -wle'use re "debug"; $x = "foo\nbar"; qr/(?x)hullo (?#there $x)
+/'
Compiling REx `(?x)hullo (?#there $x)'
size 4 Got 36 bytes for offset annotations.
first at 1
   1: EXACT <hullo>(4)
   4: END(0)
anchored `hullo' at 0 (checking anchored isall) minlen 5 
Offsets: [4]
        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)"'
[download]

Looks like it is. Thanks again for the explanation. I know now what to avoid.

Cheers,

JohnGG