Unfortunately, once you create the string '100' from 0x100 (=256), you need to be sure you are only using it as a string, not as a number, so that nothing interprets it as the number 10*10. But never fear, perl has the tools to let you create a variable that has distinct string and numeric values:

$ perl -wlne'eval;print$@if$@ # interactive perl'
use Scalar::Util 'dualvar';
$x = dualvar(256, sprintf "%x", 256);
print $x;
100
# correct hexadecimal string
print 0+$x;
256
# correct numeric value
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But there is a problem with that. If some well-meaning code tries to change our value, we lose either the string or numeric value:

$x .= "foo";
print 0+$x;
Argument "100foo" isn't numeric in addition (+) at (eval 9) line 1, <>
+ line 8.
100
print $x;
100foo
$x = dualvar(256, '100');
$x += 3;
print 0+$x;
259
print $x
259
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Kind of makes you want some way to turn on a readonly flag for a scalar you've set up this way.
require 5.008; Internals::SvREADONLY($x, 1)