Is this what you mean?

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

my $ending = "txt";

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

while (!-e "$fileName")

    {
        print STDERR "$fileName not found. Try again, or quit (q): ";
        $fileName = <STDIN>;
        chomp $fileName;
        exit () if $fileName eq "q";
    }

my @name = split ('\.', $fileName);
$name[$#name] = $ending if @name >= 2;
push(@name, $ending) if @name == 1;
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";
    }
[download]

-Michael