Perl - Command line input and STDIN - perl

Question

I've created this script below for a assignment I have. It asks for a text file, checks the frequency of words, and lists the 10 words that appear the most times. Everything is working fine, but I need this script to be able to start via the command line as well as via the standard input.
So I need to be able to write 'perl wfreq.pl example.txt' and that should start the script and not ask the question for a text file. I'm not sure how to accomplish this really. I think I might need a while loop at the start somewhere that skips the STDIN if you give it the text file on a terminal command line.
How can I do it?
The script
#! /usr/bin/perl
use utf8;
use warnings;
print "Please enter the name of the file: \n" ;
$file = <STDIN>;
chop $file;
open(my $DATA, "<:utf8", $file) or die "Oops!!: $!";
binmode STDOUT, ":utf8";
while(<$DATA>) {
tr/A-Za-z//cs;
s/[;:()".,!?]/ /gio;
foreach $word (split(' ', lc $_)) {
$freq{$word}++;
}
}
foreach $word (sort { $freq{$b} <=> $freq{$a} } keys %freq) {
#fr = (#fr, $freq{$word});
#ord = (#ord, $word);
}
for ($v =0; $v < 10; $v++) {
print " $fr[$v] | $ord[$v]\n";
}

Answer 1

Instead of reading from <STDIN>, you can read from <> to get data either from files provided on the command line or from stdin if there are no files.
For example, with the program:
#!/usr/bin/env perl
while (<>) {
print $_;
}
The command ./script foo.txt will read and print lines from foo.txt, while ./script by itself will read and print lines from standard input.

Answer 2

You need to do the following:
my $DATA;
my $filename = $ARGV[0];
unless ($filename) {
print "Enter filename:\n";
$filename = <STDIN>;
chomp $filename;
}
open($DATA, $filename) or die $!;
Though I have to say, user-prompts are very un-Unix like.

Answer 3

perl script.pl < input.txt
The use of the operator < passes input.txt to script.pl as standard input. You can then skip querying for the filename. Otherwise, use $ARGV[0] or similar, if defined.

Answer 4

You can check for a command-line argument in #ARGV, which is Perl's array that automagically grabs command line arguments, and --if present-- process them (else continue with input from STDIN). Something like:
use utf8;
use strict; #Don't ever forget this! Always, always, ALWAYS use strict!
use warnings;
if(#ARGV)
{
#Assume that the first command line argument is a file to be processed as in your original code.
#You may or may not want to care if additional command line arguments are passed. Up to you.
}
else
{
#Ask for the filename and proceed as normal.
}
Note that you might want to isolate the code for processing the file name (i.e., the opening of DATA, going through the lines, counting the words, etc.) to a subroutine that you can easily call (perhaps with an argument consisting of the file name) in each branch of the code.
Oh, and to make sure I'm clear about this: always use strict and always use warnings. If you don't, you're asking for trouble.