The while (<>) {} idiom is useful when you want your script to process data from a number of files, or from STDIN (piped, redirected, typed) and have it treated in the same way regardless. The diamond operator is provided as a shortcut. If the shortcut doesn't work for you then you will need to work with explicitly opened file handles.

Modules such as Getopt::Long extract named command line arguments and remove them from @ARGV so that you are able to work with whatever remains in @ARGV. A typical script will use Getopt::Long, extract special parameters, work with what's left. E.g.

#! /usr/bin/perl
#
# A simple file catenation
#
#

use strict;
use warnings;

use Getopt::Long;

my $target;
my $debug;

# Get optional args first
my $result = GetOptions ("target=s"   => \$target,      # string
                         "debug"  => \$debug);  # flag

# The target filename is the first filename if it hasn't already been 
+set.
$target = shift @ARGV unless $target;

open TARGET, ">$target" or die "unable to open $target [$!]\n";

foreach my $src (@ARGV)
{
    open SRC, "<$src" or die "unable to open $src [$!]\n";
    
    while (<SRC>)
    {
        # Do something with the files.
        print TARGET;
        print if $debug;
    }
    close SRC;
}

close TARGET
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