my $filename = shift // die "Usage: $0 <filename | ->\n";

my $fh = *STDIN;
if (filename ne '-') {
    open $fh, '<', $filename or die "Can't open $filename: $!";
}

while (<$fh>) {
    # Process lines of input
}
##</code><code>##

use 5.012;
use warnings;
use autodie;

my $filename = shift // die "Usage: $0 <filename | ->\n";

open STDIN, '<', $filename if $filename ne '-';

while (<STDIN>) { ... }