Here's an sprintf approach that avoids regex fireworks:

Win8 Strawberry 5.30.3.1 (64)  Mon 08/30/2021 18:36:38
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

my @n_list = (0, 1, 255, 256, 65535, 65536, 16777215, 16777216, 429496
+7295,);

sub n2hstr_2 {  # compatible with perl version 5.8
    return
        join ' ',
        map {
            my $w = $_ > 65535 ? 8 : $_ > 255 ? 4 : 2;
            unpack '(a2)*', sprintf '%0*x', $w, $_;
            }
        @_
        ;
    }

sub number2hexString {
        my $output;
        my $packTemplate;
        foreach my $i (@_) {
                if ($i > 65535) {
                        $packTemplate = 'L>';
                } elsif ($i > 255) {
                        $packTemplate = 'S>';
                } else {
                        $packTemplate = 'C';
                }
                $output .= join(' ', unpack('(H2)*', pack($packTemplat
+e, $i))).' ';
        }
        return $output;
}

my $n_n2hstr           = n2hstr_2        (@n_list);
my $n_number2hexString = number2hexString(@n_list);

print "@n_list \n";
print "'$n_n2hstr' \n";
print "'$n_number2hexString' \n";

^Z
0 1 255 256 65535 65536 16777215 16777216 4294967295
'00 01 ff 01 00 ff ff 00 01 00 00 00 ff ff ff 01 00 00 00 ff ff ff ff'
'00 01 ff 01 00 ff ff 00 01 00 00 00 ff ff ff 01 00 00 00 ff ff ff ff 
+'
[download]

As you see, the outputs are the same — except that number2hexString() adds an extra space at the end of the output string! I don't attempt to handle negative numbers since I don't yet understand how they should be handled.

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