It is all in the docs

$ perldoc -f sprintf
:
%X    like %x, but using upper-case letters
:
0     use zeros, not spaces, to right-justify
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G'day mandalmanas5519,

Welcome to the monastery.

The documentation for sprintf has dozens of examples of how to compose the format and what the output looks like. So, for future reference, looking it up there would be a lot quicker than posting a question here and waiting for a reply.

Here's how to compose the format for the specific instance you have here.

1. % — start format (yes, you know this already)
2. %0 — padding with zeros
3. %08 — 8 characters
4. %08X — uppercase hex digits (you don't need uc)

$ perl -Mstrict -Mwarnings -E '
    my $dec = 1211;
    say sprintf("%08x", $dec);
    say sprintf("%08X", $dec);
'
000004bb
000004BB
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Please use code tags around any example code, it will make your questions easier to read.

Here is a slight tweak of your code that should do what you want.

use strict;
use warnings;

my $dec = 1211;

print "Hexadecimal number: ", uc(sprintf("%08x\n", $dec)), "\n";
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