The docs for readline are wrong for two reasons:

• As previously stated, $! can't be used to determine if an error occured.
• The default handle ARGV already handles errors, so readline doesn't see them.

It's on my todo list to fix the documentation.

For handles other than ARGV, you want

while (!eof($fh)) {
    defined( $_ = <$fh> )
        or die("Can't readline: $!\n");

    ...
}
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