I'll use \U as an example, because it's slightly less mind-bending to follow what's going on. But the same thing applies to all three directives (and it's really only \Q that I'm really interested in).

Consider:

all is well and good, but what if you want to fetch the pattern from the command line?

perl -le '$patt = shift; print qr/$patt/' 'a\Ubc'
# prints (?-xism:a\Ubc)

perl -le '$patt = shift; print qr/$patt/' 'a\\Ubc'
# prints (?-xism:a\\Ubc)
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I.e., I tried doubling up the backslashes just in case the shell was giving me grief, but that's not the case. And regardless of that, I don't particularly care what it looks like, the main issue is that it doesn't match what it should:

my $patt = shift; # e.g. 'a\Ubc' from the shell
$patt = qr/$patt/;
my $target = 'aBC';
print $target =~ /$patt/; # prints nothing
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Now this doesn't match aBC. It doesn't match 'a\Ubc' literally, either for that matter. In fact, I don't know what, if anything, it does match.

I have figured out one way to make it work: put the qr// expression inside a string eval and all is well:

my $patt = shift;
$patt = eval "qr/$patt/"; # eeeww
# patt is now (?-xism:aBC) if given 'a\Ubc'
my $target = 'aBC';
print $target =~ /$patt/; # prints 1
[download]

Now all is fine, but the cure is worse than the disease. Any person reading the code will quickly spot that they could have a lot of fun by specifying a pattern such as /.`rm -rf /`./ and then you are in a world of pain.