Another approach:

Win8 Strawberry 5.8.9.5 (32)  Sun 08/29/2021 18:20:29
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

for my $n (qw(2 20 200 2000 20000 200000 2000000)) {
    print "$n -> ";
    print join ' ', unpack '(H2)*', pack 'V', $n;
    print "\n";
    }

^Z
2 -> 02 00 00 00
20 -> 14 00 00 00
200 -> c8 00 00 00
2000 -> d0 07 00 00
20000 -> 20 4e 00 00
200000 -> 40 0d 03 00
2000000 -> 80 84 1e 00
[download]

As with LanX's solution, leading | trailing zeros are present, but they are present in some of the OPed examples as well and I can't figure out when you do and don't want them.

Update: If you're using a 64-bit Perl, you have the 'Q' pack template specifier (update: which uses native endianity by default; see below), so:

Win8 Strawberry 5.30.3.1 (64)  Sun 08/29/2021 20:05:59
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

for my $n (qw(2 20 200 2000 20000 200000 2000000)) {
    print "$n -> ";
  # print join ' ', unpack '(H2)*', pack 'Q', $n;  # original
    print join ' ', unpack '(H2)*', pack 'Q<', $n;
    print "\n";
    }

^Z
2 -> 02 00 00 00 00 00 00 00
20 -> 14 00 00 00 00 00 00 00
200 -> c8 00 00 00 00 00 00 00
2000 -> d0 07 00 00 00 00 00 00
20000 -> 20 4e 00 00 00 00 00 00
200000 -> 40 0d 03 00 00 00 00 00
2000000 -> 80 84 1e 00 00 00 00 00
[download]

(but you get a lot more trailing zeroes :). (Update: The 'Q' specifier should be used with a '<' little-endian specifier to force the desired byte ordering. 'Q' alone - as used in my originally posted code - uses native byte ordering, whatever that may be.)