#!/usr/bin/perl 
use strict;
use warnings;

print STDERR "Enter filename: ";
my $fileName = <STDIN>;
chomp $fileName;

die "$fileName not found.\n\n" unless -e $fileName;

my @name = split ('\.', $fileName);
$name[$#name] = "txt";
my $newFileName = join(".", @name);

print STDERR "\nOLD: $fileName\nNEW: $newFileName\n\n";

open (IN, "<", $fileName);
open (OUT, ">", $newFileName);

while (<IN>)
	{
		#do stuff...
		#print OUT "output";
	}