my $line;
while ($line = <>) {
    ...
[download]

With the declaration inside the condition, it's in fact a different variable every time, so Perl needs to create an extra scope for it, as B::Deparse shows you:

$ perl -MO=Deparse -e 'while (<>) { /(.)/ }'
while (defined($_ = readline ARGV)) {
    /(.)/;
}
-e syntax OK

$ perl -MO=Deparse -e 'while (my $line = <>) { $line =~ /(.)/ }' 
while (defined(my $line = readline ARGV)) {
    do {
        $line =~ /(.)/
    };
}
-e syntax OK
[download]

Also note that lines containing 0 as their first or second word are not printed.