I hope this is not too dumb a question :-) While working on a dictionary module, I came up with the following question: Why is $_ not set in a while loop like the following one? I would expect it to work like while (<>) { # do stuff }

while (match('a')) {
    print "- $_\n";   # $_ is undef all the time
}

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I can fix it using a loop variable, but it doesn't look as sexy and it doesn't help me to understand why the first option doesn't work.

while (my $entry = match('a')) {
    print "- $entry\n";  # prints the correct results
}

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Here's an example match function to let you understand what it's all about. When having searched all entries, the function returns undef. (Note that in the original problem match returns a reference, but the problem is the same.) 
my $matchPos = 0;

sub match {
    my ($key) = @_;
    
    while ($matchPos < @data) {
        $matchPos++;
        return $data[$matchPos - 1] 
            if ($data[$matchPos - 1] =~ /$key/);
    }
    
    return undef;
}

while (match('a')) {
    print "- $_\n";
}

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In reply to When does while set $_ ? by crenz

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