$ perl -le '$r = qr/qux./; $s = "qux\n"; print "matches" if $s =~ /$r/'
$ perl -le '$r = qr/qux./; $s = "quxxx"; print "matches" if $s =~ /$r./'
matches
$ perl -le '$r = qr/qux./; $s = "quxx\n"; print "matches" if $s =~ /$r./'
$ perl -le '$r = qr/qux./; $s = "quxx\n"; print "matches" if $s =~ /$r./s'
matches