open(TXTIN,"ARGV[0]") || die "Cannot open the data file";
open(TXTOUT,">ARGV[1]") || die "Cannot open the formatted file";
while(<TXTIN>){                    # gets a line from input file
  if( eof TXTIN ){                 # eof returns true if the _next_ read would fail, meaning
    s/different/regex here/;       #    the one we just read was the last line
  }else{
    s/yada/yadda/;
  }
  print TXTOUT $_;
}
close(TXTIN);
close(TXTOUT);