use File::ReadBackwards ()

my $fh_in = File::ReadBackwards->new($ARGV[0])
   or die("Unable to open the data file: $!\n");

open(my $fh_out, '>', $ARGV[1])
   or die("Unable to create output file: $!\n");

local $_;

my $last = 1;

while (defined($_ = $fh_in->readline())) {
   if ($last) {
      s/different/regex here/;
   } else {
      s/yada/yadda/;
   }

   print $fh_out $_;

   $last = 0;
}
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