Returns the number of characters actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set).

my $r;
while ($r = read ...) {
    ... do stuff here ...
}
die "read failed: $!" unless defined $r;
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while (read(...) // die "read failed: $!") {
    ... do stuff here ...
}
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