Or the semi-obfuscated version: as long as you just emptied out $/, why not use it? :)

{
  local $/;
  $/ .= $_ while <STDIN>;
}
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P.S. In general, you want to make changes to global variables a temporary condition, as in merlyn's do loop above, and the more general form:

{
  local $/; ## This is already undef, no need to set it
  ## Read in the file, etc...
}
## $/ is now back where it was
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