qr// returns an object of the class Regexp. You can make methods for them like any other object. In addition, Perl knows that if it sees /$foo/, and $foo is an object of Regexp, it doesn't need to compile the regex, it uses the information stored in the object.

jpinyan@sushi [2:02pm] ~ #501> perl -MDevel::Peek -e 'Dump(qr/a+bc?/)'
SV = RV(0xf18d0) at 0xe46f0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0xe4798
  SV = PVMG(0xf0a78) at 0xe4798
    REFCNT = 1
    FLAGS = (OBJECT,RMG)
    IV = 0
    NV = 0
    PV = 0
    MAGIC = 0xf2158
      MG_VIRTUAL = 0xe336c
      MG_TYPE = 'r'
      MG_OBJ = 0xf0ef0
    STASH = 0xef128     "Regexp"
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