#!/usr/bin/perl
$Inputfile='filename';
open (IN,"<$Inputfile") or die "cannot open file: $!\n";
open (OUT, "+<$inputfile.out") or die "cannot open file: $!\n";
while (<IN>){
....
.....
print OUT;  #print to $Inputfile.out file
}
#do something with content of $Inputfile.out