$ perl -wle'use re "debug"; $x = "foo\nbar"; qr/(?x)hullo #there $x/'
Compiling REx `(?x)hullo #there foo
bar'
size 4 Got 36 bytes for offset annotations.
first at 1
   1: EXACT <hullobar>(4)
   4: END(0)
anchored `hullobar' at 0 (checking anchored isall) minlen 8
Offsets: [4]
        5[20] 0[0] 0[0] 25[0]
Freeing REx: `"(?x)hullo #there foo)\nbar"'