If no files are specified on the command line, print prints the standard input
Possible problem:
The standard input may need to be converted to another format
Typically, this means that print will examine the first bit of input
Then decide what format it is in
Then call another program to convert the formats
But examining the first bit of input destroys it!
Solution: copy STDIN to a file first
unless (@ARGV) {
push @ARGV, save_stdin();
}