Programmatically read from STDIN or input file in Perl

Question

What is the slickest way to programatically read from stdin or an input file (if provided) in Perl?

Answer 1

You need to use <> operator:

while (<>) {
    print $_; # or simply "print;"
}

Which can be compacted to:

print while (<>);

Arbitrary file:

open F, "<file.txt" or die $!;
while (<F>) {
    print $_;
}
close F;

Answer 2

while (<>) {
print;
}

will read either from a file specified on the command line or from stdin if no file is given

Answer 3

if(my $file = shift) { # if file is specified, read from that
  open(my $fh, '<', $file) or die($!);
  while(my $line = <$fh>) {
    print $line;
  }
}
else { # otherwise, read from STDIN
  print while(<>);
}