my $file_r = $ARGV[0];
my $file_r = $ARGV[1];
open FILE_R, "< $file_r" or die "Can't open $file_r : $!";
open FILE_P, "< $file_p" or die "Can't open $file_p : $!";
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while (<FILE_R>){
  chomp;
  push @rdata, $_;
}

while (<FILE_R>){
  chomp;
  push @rdata, $_;
}

my @sum = map {$rdata[$_] + $pdata[$_]} 0..$_#a;
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Please note that while it's easy to understand what your code could be supposed to do, the above simply won't compile, and it's advisable to post minimal, but complete, working samples of code.

As a side note, since basically you trust your files to hold the same number of entries, you could avoid putting all of their contents into arrays to process them later (this basically amounts to -unnecessary- slurping) and process them sequentially instead...