Also, it isn't necessary for $_ to be undefined (or unassigned) for the loop to end.

While that's true in general,

>perl -le"$_=4; while (--$_) { print }"
3
2
1
[download]

It's not true in this case.

>perl -e"print 0" | perl -le"while ($_ = <>) { print $_ ? 'true' : 'fa
+lse' }"
false

>perl -MO=Deparse -e"while (<>) {}"
while (defined($_ = <ARGV>)) {
    ();
}
-e syntax OK

>perl -MO=Deparse -e"while ($_ = <>) {}"
while (defined($_ = <ARGV>)) {
    ();
}
-e syntax OK

>perl -MO=Deparse -e"while (defined($_ = <>)) {}"
while (defined($_ = <ARGV>)) {
    ();
}
-e syntax OK
[download]