Here is an example I wrote just to show you, so it doesn't actually do anything. However it outputs the warning produces by the pragma diagnostics (so I know that if I would get rid of the pragma, I would get rid of the message, but is that moral :-) )

Here is the code

#!/usr/bin/perl -w

use strict;
use warnings;
use diagnostics;
use Benchmark;
use Getopt::Long qw(:config no_ignore_case bundling);

#defines variables
my $man = 0;
my $help = 0;
my $input;
my $output;
my @blubb;

my %options = (
    'help|h'     => \$help,
    'man'        => \$man,
    'input|i=s'  => \$input,
    'output|o=s'  => \$output,
  );

GetOptions(%options);

open STDIN, "<$input" or die "Can't open $input: $!" if $input;
while(<STDIN>)
{
  chomp;
  push @blubb, $_;
}
close STDIN if $input;

open STDOUT, ">$output" or die "Can't open $output: $!" if $output;
print join "\n", @blubb;
print "\n";
close STDOUT if $output;

exit 0;
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And here is the error message

Filehandle STDIN reopened as STDOUT only for output at ex.pl line 36 (
+#1)
(W io) You opened for writing a filehandle that got the same filehandl
+e id
as STDIN. This occured because you closed STDIN previously.
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