Will the whole thing fit in memory? Just read from the filehandle in list context...

  my @input = <STDIN>;
  for (@input) {
     process_line($_);
  }
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Now your Perl script can take as long as it likes to process the data; the other program has finished and probably exited, but so what?

$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
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