Let's say, you have the following program:

#!/usr/bin/perl -T

use strict;
use warnings;

my $file = shift;

open my $fh => $file or die $!;
while (<$fh>) {print}
close $fh;

__END__
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A fairly trivial one. Takes only one parameter. You test it with a million files. It all works fine. You remove the '-T' and put it in production, where it's going to be run suid or called by a CGI program, or whatever.

Then some joker passes "> /some/important/file" as argument. With "-T", perl would not have wiped the content of the file. Without, it will.

Abigail