Hello GrandFather,

Well, according to perlsyn#Compound-Statements, “The while statement executes the block as long as the expression is true.” So far, so good. But if that were the whole story, this:

#! perl
use strict;
use warnings;

print "Contents of \$/: |$/|\n";
print ">$_<\n" while <DATA>;

__DATA__
7
42
0
[download]

would print only the first two lines, since the final line — which is not terminated by a newline — is '0' and therefore false. But the final line is printed, because with the <> operator (or its equivalent readline call) the while condition is tested for definedness, not truth. This is documented in perlop#IO/Operators:

In these loop constructs, the assigned value ... 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.

All of which is (apparently) undercut by the op’s assertion, below, that adding an explicit test for definedness solved his problem. I don’t know how to explain that. An early version of Perl? A strange value for $/? I do know that perlop#IO/Operators says the following are equivalent:

while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
[download]

and immediately goes on to imply that explicit assignment to a lexical variable “behaves similarly” to implicit assignment to the default variable, $_.