I think your basic question is solved, and ysth has warned you about a possible risk with your open() statements. I have a slightly different admonishment about getting the file names (and min/max params):

Use @ARGV, and get the file names, and min/max values as well, from the command line, instead of forcing the user into a dialog with printed prompts and keyboard input via STDIN. You can look at the Getopt::Long and Getopt::Std modules to see how most people do this sort of thing, or you can easily "roll your own" -- something like this:

my $Usage = "Usage: $0 --min MIN --max MAX --in INFILE --out OUTFILE\n
+";

my %args;
while ( @ARGV >= 2 ) {
   if ( $ARGV[0] =~ /^--(min|max|in|out)$/ ) {
      my $opt = substr( shift, 2 );
      $args{$opt} = shift;
   } else {
      last;
   }
}
die $Usage if ( @ARGV or ( keys %args != 4 ));
# all 4 args are mandatory, and only these 4 args are allowed

# now use $args{in} instead of $input, etc...
[download]

(But really, the Getopt modules are worthwhile -- reading their man pages is time well spent.)

There are several reasons for using command-line args in @ARGV instead of a runtime dialog with the user:

• Most command line shells provide a "command history" feature that lets you recall, modify and re-execute prior commands; your program will be a lot easier and quicker to use if it gets everything it needs from the command line.
• Most shells provide pretty good line-editing functions (moving back and forth on the line before executing the command, deleting / cutting / pasting characters or words, etc) whereas the handling of STDIN within a perl script may provide very limited editing abilities (unless you learn about and use a suitable Term module).
• On the command line, you can alter/correct any arg at any time before starting execution; but if you're typing in single answers to questions at runtime and notice a mistake after entering any one of the answers, you just have to quit and start over (very frustrating).
• If the program gets everything it needs from the command line, you can "automate" it -- i.e. run it from some other script; but if you have to carry on a dialog after it starts, running it via some other script becomes a fairly complicated and tricky business.
• (update -- forgot to mention:) Many shells provide "file name completion" on the command line, whereby you simply type the first few characters of a name, hit the tab key or something, and the rest of the file name is filled in for you (or you get a list of files that match what you've typed so far); again, you would only get this feature within a perl script if you learned and used one of the Term modules (which I haven't done yet, because @ARGV handles pretty much everything I ever need).

Note that the ordering of arg flags is flexible: "--min 0" can come before or after "--max 10", and likewise for the file name args; all that matters is that each value comes immediately after its flag.

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