\t          tab                   (HT, TAB)
\n          newline               (LF, NL)
\r          return                (CR)
\f          form feed             (FF)
\a          alarm (bell)          (BEL)
\e          escape (think troff)  (ESC)
\033        octal char            (example: ESC)
\x1B        hex char              (example: ESC)
\x{263a}    long hex char         (example: Unicode SMILEY)
\cK         control char          (example: VT)
\N{name}    named Unicode character
\l          lowercase next char (think vi)
\u          uppercase next char (think vi)
\L          lowercase till \E (think vi)
\U          uppercase till \E (think vi)
\E          end case modification (think vi)
\Q          quote (disable) pattern metacharacters till \E
##</code><code>##

my $f = 2;
print qr/$f/,"\n"; # (?-xism:2)
print qr'$f',"\n"; # (?-xism:$f)