The track number available in an ID3v1 tag is a small unsigned integer number stored in a single byte. If you retrieve this information once MP3::Tag has stored it in such a tag you will only get a numerical value back, without fancy formatting.
in reply to MP3::Tag - Leading zeros on 'track' truncated
The documentation you quote refers to parse_rex() -- a method not related to your code.
If you want to find out about the conversion from a Perl scalar holding some numerical value into a raw byte look out for the pack function.
$x = pack 'C', 65;
print "$x\n"; # prints the letter A
$n = unpack 'C', $x;
print "$n\n"; # prints the number 65