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

##</code><code>##

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

##</code><code>##

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";
}