To understand how precedence works in this circumstance, consider the following variation of the OP's code. The assignment operator has enough precendence to end evaluation of the trinary conditional operator, so the conditional operator is simply choosing an "lvalue" for the assignment operator. The parenthesis in the second loop help show what's actually happening.

use strict;
use warnings;

my $x='baz';
my $y='moo';

for (1..10) {
        $_>5 ? $x='foo' : $y='bar';
        print "x=$x, y=$y\n";
}

# re-initialize vars
print "----------\n";
$x = 'baz'; 
$y = 'moo';

# the above loop is functionally equivalent to:
for (1..10) {
        ($_>5 ? $x : $y) = 'bar';
        print "x=$x, y=$y\n";
}
[download]

Output:

x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
----------
x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=baz, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
x=bar, y=bar
[download]

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