perlsec tells us the following:

The exception to the principle of "one tainted value taints the whole
expression" is with the ternary conditional operator "?:".  Since code
with a ternary conditional

  $result = $tainted_value ? "Untainted" : "Also untainted";

is effectively

  if ( $tainted_value ) {
    $result = "Untainted";
  } else {
    $result = "Also untainted";
  }

it doesn't make sense for $result to be tainted.
[download]

However, this does not appear to be the whole story because those two constructions are not in fact identical as the following script demonstrates

#!/usr/bin/perl -T
use warnings;
use strict;
use Scalar::Util qw/tainted/;

# Note: $t is tainted
my $t = "purple" . substr($^X,0,0);

my $foo = "blah";
my $bar = "foo: $foo";

my $one;
$one = $t ? "the $foo" : $bar;
print "1. tainted\n" if tainted($one);

my $two;
$two = $t ? "the foo" : $bar;
print "2. tainted\n" if tainted($two);

my $three;
if ($t) {
  $three = "the $foo";
} else {
  $three = $bar;
}
print "3. tainted\n" if tainted($three);

my $four;
if ($t) {
  $four = "the foo";
} else {
  $four = $bar;
}
print "4. tainted\n" if tainted($four);

print "5. tainted\n" if tainted($bar);
print "6. tainted\n" if tainted("the $foo");

my $seven = "";
$seven = "the $foo" if $t;
print "7. tainted\n" if tainted($seven);
[download]

The above script prints

1. tainted
7. tainted
[download]

in perl 5.8.8 and perl 5.10.0.

For one I find edge cases like this rather interesting. But I also I wonder is whether this should be filed as a bug and whether it should be filed as a perl bug or a documentation bug.

Update: Point out that $t is tainted.
Update: Add tests 5, 6, and 7.
Update: Filed bug 59916

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