use Modern::Perl;
use autodie;

my $infile  = '/Users/ts/desktop/work/numbers.txt'; # Hmm...
my $outfile = 'Users/ts/full_number.txt';           # leading '/' or not?

open my $in, '<', $infile;

my @result;
while ( <$in> ) {
    my ( $index, $value ) = split;
    $result[$index] = $value;
}

close $in;

open my $out, '>', $outfile;
say $out "$_\t", $result[$_] || 0 for 1 .. 3_000_050;
close $out;