my %b_lookup;
for (<$file_b>) {
        my ($num, $data) = split(/ /, $_, 2);
        $b_lookup{$num} = "$num $data";
}

for (<$file_a>) {
        my ($num, $data) = split(/ /, $_, 2);
        
        if (exists $b_lookup{$num}) {
                print $file_c $b_lookup{$num}."\n";
        }
        else {
                print $file_c "$_\n";
        }
}