You are "sabotaging" one of the main use cases for the ternary conditional operator : to return an (l)value. ¹ ²

Better use if-then-else construct instead, if you don't want to take advantage, but execute complex code.

 
my $z;
if ($x) {$z  = "true\n"} else {$z  = "false\n"}
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But for my taste it's better written as

 
my $z = $x ? "true\n" : "false\n";
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Your trap is even documented in perlop#Conditional-Operator

Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:

$x % 2 ? $x += 10 : $x += 2

Really means this:

(($x % 2) ? ($x += 10) : $x) += 2

Rather than this:

($x % 2) ? ($x += 10) : ($x += 2)

That should probably be written more simply as:

$x += ($x % 2) ? 10 : 2;

¹) See also WP : "The conditional operator's most common usage is to make a terse simple conditional assignment statement. "

²) tho I don't think I ever took advantage of the lvalue aspect, to create a LHS

  DB<22> 1 ? $a : $b = 1

  DB<23> x $a,$b
0  1
1  undef
  DB<24> 0 ? $a : $b = 2

  DB<25> x $a,$b
0  1
1  2
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(I'd certainly use parentheses in productive code.)