% perl -MO=Deparse,-p -le '$patt = "a\Ubc"; print qr/$patt/'
BEGIN { $/ = "\n"; $\ = "\n"; }
($patt = 'aBC');
print(qr/$patt/);
-e syntax OK

##</code><code>##

% perl -MO=Deparse,-p -le '$patt = "a\Q[bc]\E"; print qr/$patt/'
BEGIN { $/ = "\n"; $\ = "\n"; }
($patt = 'a\\[bc\\]');
print(qr/$patt/);
-e syntax OK

##</code><code>##

% perl -le '$patt = q(a\Ubc); print qr/$patt/'
(?-xism:a\Ubc)