I have some issues with your indenting.
It is not allowed to optionally create a variable in an if statement.
I would strongly advise to "use strict;"
I suggest something like the following... 
#!/usr/bin/perl
use warnings;
use strict;
use File::Basename;

die "usage: $0 in_file\n" if @ARGV !=1;
my $in_file = shift @ARGV;

my($filename, $dir, $suffix) = fileparse($in_file);
my $out_file = "$dir/$filename.out"; #change the suffix

open my $FH_infile, '<', "$in_file" or die "Can't open $in_file";
open my $FH_outfile, '>', "$out_file" or die "Can't open $out_file";

[download]
I didn't run this code, but I think it is a good general idea...

In reply to Re: First attempt at bringing in file for input/output by Marshall
in thread First attempt at bringing in file for input/output by catfish1116

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.