It seems perl applies its magic test for definedness to any while that involves a filehandle (at least, I couldn't find a case where it didn't apply). 
$ perl -MO=Deparse -e 'while (<>) {} while ( <ARGV> ) {} while ( $_ = 
+<> ) {} while ( $x = <> ) {}'
while (defined($_ = <ARGV>)) {
    ();
}
while (defined($_ = <ARGV>)) {
    ();
}
while (defined($_ = <ARGV>)) {
    ();
}
while (defined($x = <ARGV>)) {
    ();
}
-e syntax OK

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Unless I state otherwise, all my code runs with strict and warnings

In reply to Re^3: Doubt on defined-ness by FunkyMonk
in thread Doubt on defined-ness by waldner

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