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for(my $x = 0; $x <= $#array; $x++) {
    next unless $array[$x];
    # do stuff with $x and $array[$x]
}

my %hash = 
    map { $x => $y } 
    grep { my $y = $list[$x]; $y ne $undef; [$x, $y] }
    for(my $x = 0; $x <= $#list; $x++);

$list{100} = 'dog';
$list{435} = 'cat';
$list{1040} = 'tax form';
...
while(($index, $element) = each %list) {
    # do stuff with $index and $element
}