I wonder about "(OBJECT,FAKE)" and why "MOTHER_RE" is there, and so maybe with pre-compiled qr Perl has to follow this chain to top mother each time it tries to match.

>perl -MDevel::Peek -E "Dump qr/x/"
SV = IV(0x2fda530) at 0x2fda540
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x2fda588
  SV = REGEXP(0x266a378) at 0x2fda588
    REFCNT = 1
    FLAGS = (OBJECT,FAKE)
    PV = 0x26d66a8 "(?^u:x)"
    CUR = 7
    STASH = 0x2656a50   "Regexp"
    COMPFLAGS = 0x100 ()
    EXTFLAGS = 0x680100 (CHECK_ALL,USE_INTUIT_NOML,USE_INTUIT_ML)
    ENGINE = 0x6e1a9ee0 (STANDARD)
    INTFLAGS = 0x0 ()
    NPARENS = 0
    LASTPAREN = 0
    LASTCLOSEPAREN = 0
    MINLEN = 1
    MINLENRET = 1
    GOFS = 0
    PRE_PREFIX = 5
    SUBLEN = 0
    SUBOFFSET = 0
    SUBCOFFSET = 0
    SUBBEG = 0x0
    MOTHER_RE = 0x26626f8
    SV = REGEXP(0x267aff0) at 0x26626f8
      REFCNT = 2
      FLAGS = ()
      PV = 0x26d66a8 "(?^u:x)"
      CUR = 7
      COMPFLAGS = 0x100 ()
      EXTFLAGS = 0x680100 (CHECK_ALL,USE_INTUIT_NOML,USE_INTUIT_ML)
      ENGINE = 0x6e1a9ee0 (STANDARD)
      INTFLAGS = 0x0 ()
      NPARENS = 0
      LASTPAREN = 0
      LASTCLOSEPAREN = 0
      MINLEN = 1
      MINLENRET = 1
      GOFS = 0
      PRE_PREFIX = 5
      SUBLEN = 0
      SUBOFFSET = 0
      SUBCOFFSET = 0
      SUBBEG = 0x0
      MOTHER_RE = 0x0
      PAREN_NAMES = 0x0
      SUBSTRS = 0x26b20b8
      PPRIVATE = 0x26c16e8
      OFFS = 0x26c3878
      QR_ANONCV = 0x0
      SAVED_COPY = 0x0
    PAREN_NAMES = 0x0
    SUBSTRS = 0x26b3238
    PPRIVATE = 0x26c16e8
    OFFS = 0x26c3ab8
    QR_ANONCV = 0x0
    SAVED_COPY = 0x0
[download]

p.s. BTW, to answer 1205970, $re is compiled once, either checking debug output, or "hoisting" first line of subroutine to top of the script, performance remains the same.