$ cat test1.pl
#!/usr/bin/perl

use strict;
use warnings;

while (<>){
  print;
}

$ cat > testfile
two
three

$ echo "one" | perl test1.pl testfile
one
two
three

$ cat test2.pl
#!/usr/bin/perl

use strict;
use warnings;

my $file = shift @ARGV;

my $ifh;
my $is_stdin = 0;
if (defined $file){
  open $ifh, "<", $file or die $!;
} else {
  $ifh = *STDIN;
  $is_stdin++;
}

while (<$ifh>){
   print
}
close $ifh unless $is_stdin;

$ echo "one" | perl test2.pl testfile
two
three


##</code><code>##

unshift(@ARGV, '-') unless @ARGV;
    while ($ARGV = shift) {
	open(ARGV, $ARGV);
	while (<ARGV>) {
	    ...		# code for each line
	}
    }