That usage of <> is a shorthand for readline. If you switch to readline it still adds to the defined check and the assignment to $_.

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

$ perl -MO=Deparse -e'while (readline($fh)) {}'
while (defined($_ = <$fh>)) {
    ();
}
-e syntax OK
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