use warnings;
use strict;

reader($_) for (@ARGV);

sub reader {
    my ($filename) = @_;

    open my $fout, '>>', 'output_file.txt' or die "can't open file: $!";
    open my $fh,   '<',  $filename         or die "can't open file: $!";
    while (<$fh>) {
        print $fout $_;
    }

    print $/;
}