If you're undef'ing it, then you're in slurp mode, and there's no point in looping, because there's only going to be one line in the file ...

sub slurpie {
      local $/  ;
      return <DATA>;  # contains whole thing
}

sub DATA_w_line_numbers {
      while (<DATA>) {
           print "$.) $_";
      }
}
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Also be sure to read through all off the perlvar section on $/ -- i think you're looking for local $/ = "";

This may be of interest as well: Perl Idioms Explained - my $string = do { local $/; <FILEHANDLE> };