Greetings.

I would like to understand when $_ gets a value assigned and when it doesn't. The question comes up in code like:

use strict;
use warnings;

while (1) {
    <STDIN>;
    m/./ and print "success.\n";
    # ... (more code) ...
}
[download]

If I run it and enter a non-empty line, I get:

Use of uninitialized value in pattern match (m//) 
at uninit.pl line 6, <STDIN> line 1.
[download]

My understanding is $_ does not have a value when the pattern match is done. The following does what was intended:

use strict;
use warnings;

while (<STDIN>) {
    m/./ and print "success.\n";
    # ... (more code) ...
}
[download]

What is the difference between the two snippets with regard to $_? Why does <STDIN> in the first snippet not give $_ a value? Or should I look at it from another angle somehow to understand it?

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