% perl -e 'use diagnostics; $a, $b = <STDIN>;'
Useless use of a variable in void context at -e line 1 (#1)
(W void) You did something without a side effect in a context that does
nothing with the return value, such as a statement that doesn't return a
value from a block, or the left side of a scalar comma operator. Very
often this points not to stupidity on your part, but a failure of Perl
to parse your program the way you thought it would. For example, you'd
get this if you mixed up your C precedence with Python precedence and
said
$one, $two = 1, 2;
when you meant to say
($one, $two) = (1, 2);
Another common error is to use ordinary parentheses to construct a list
reference when you should be using square or curly brackets, for
example, if you say
$array = (1,2);
when you should have said
$array = [1,2];
The square brackets explicitly turn a list value into a scalar value,
while parentheses do not. So when a parenthesized list is evaluated in
a scalar context, the comma is treated like C's comma operator, which
throws away the left argument, which is not what you want. See
perlref for more on this.
Name "main::a" used only once: possible typo at -e line 1 (#2)
(W once) Typographical errors often show up as unique variable names.
If you had a good reason for having a unique name, then just mention it
again somehow to suppress the message. The our declaration is
provided for this purpose.
Name "main::b" used only once: possible typo at -e line 1 (#2)
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