I want to clarify something based on documentation from perlop:

while (my $line = <STDIN>) { print $line }
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In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or a "0" with no trailing newline.

So in this case the defined test doesn't need to be done explicitly, it's already being done implicitly.