I'm trying to find a way of testing for the existence of standard input in a perl program : I've done a lot of searching but so far can't find the answer. I've seen some sample code from Randal where he made a perl grep program : 
#!/bin/perl -w
$names++, shift if $ARGV[0] eq "-l";
$search = shift;
$showname = @ARGV > 1;
@ARGV = "-" unless @ARGV;
@ARGV = grep { -T or $_ eq "-" } @ARGV;
exit 0 unless @ARGV;
while (<>) {
        next unless /$search/o;
        if ($names) {
                print "$ARGV\n";
                close ARGV;
        } else {
                print "$ARGV: " if $showname;
                print;
        }
}

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I'll call the above program "grep". The program works fine for either of these 2 methods : 
1.  grep pattern /tmp/file
2.  cat /tmp/file | grep pattern

[download]
But when I run "grep" by itself, it waits for STDIN. I want the program to print out a "usage message" if the user just runs "grep". How would I do this ?

In reply to Test for standard input by Anonymous Monk

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