There are tokens that will be understood as infinity, but it's dependant on the underlying C library. It's easier just to calculate it.

use constant POS_INF => +(99**99**99);
use constant NEG_INF => -(99**99**99);
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(Is that big enough for a long double system?)

Active Perl 5.6.0, 5.6.1, 5.8.0, 5.8.8, 5.10.0 on Windows:

>perl -MDevel::Peek -e"Dump +(99**99**99)"
SV = NV(0x1832414) at 0x225334
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,NOK,READONLY,pNOK)
  NV = 1.#INF

>perl -MDevel::Peek -e"Dump -(99**99**99)"
SV = NV(0x1832424) at 0x22531c
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,NOK,READONLY,pNOK)
  NV = -1.#INF
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Some Perl 5.8.4 on linux:

$ perl -MDevel::Peek -e'Dump +(99**99**99)'
SV = NV(0x8165bd0) at 0x8160954
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,NOK,READONLY,pNOK)
  NV = inf

$ perl -MDevel::Peek -e'Dump -(99**99**99)'
SV = NV(0x8165be0) at 0x814cc9c
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,NOK,READONLY,pNOK)
  NV = -inf
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