use strict;
use warnings;

my $file = $ARGV[0];
open my $fh, '<', $file or die "cannot open $file";

# pause some second to go to inspect the process manager
sleep 5;

# the diamond op <> slurp the whole file when in list context (the array to the left force list context)
my @array_of whole_file = <$fh>;

sleep 5;

##</code><code>##

use strict;
use warnings;

my $file = $ARGV[0];
open my $fh, '<', $file or die "cannot open $file";

# pause some second to go to inspect the process manager
sleep 5;

# the diamond op is now in scalar context
while (my $line = <$fh>){
      # do noting
      1;
}

sleep 5;

##</code><code>##


foreach my $file (@ARGV){

    # check it exists..
    die "$file does not exists!" unless -e $file;

    print "start processing $file\n";

    # send the file to your own sub for processing
    my_custom_process_sub($file);

    ...