• open (INFILE, "$ARGV[1]") or die "$ARGV[1] cannot be openned : $!";
  while ($source_file =~ /(\d)\s+(\d)\s+(\d)\s+(\w+)/)
  [download]

  Maybe (probably) in your head there's a connection between INFILE and $source_file, but not in your script.
  For Perl, they're unrelated…

• if ($ARGV[0] eq "-a")
  [download]

  How do you know your $ARGV[0] really equals "-a"?
  What happens if it doesn't?
  I recommend to add an else block with a print "whatever\n", at least while you're still experimenting.
• Many of the suggestions to your questions include using strict and warnings. Although this seems to impede or slowing down one's development, I recommend it, too:
  • Yes, the time it takes before your script "runs", will be longer
  • The time until your script works correctly, will be shorter
  One of strict's messages is confusing for beginners:

  Global symbol "$x" requires explicit package name

  should read

  Do you really want "$x" to be a global symbol? Better declare it with my

If you are new to Perl, you might like diagnostics, which won't throw more errors, but messages that (hopefully) are more informative. So, your script(s) should start with

use strict;
use warnings;
use diagnostics;
[download]

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