$ perl -we '$regex = qr/foo/; print "$regex: $1\n" while "foob" =~ /$regex/g;'
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
(?^:foo): 
$ perl -we '$regex = qr/(foo)/; print "$regex: $1\n" while "foob" =~ /$regex/g;'
(?^:(foo)): foo
$