That doesn't work. Interpolation happens before the regex is performed, so your $1 refers to a previous succesful match:

#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    chomp;
    printf "%d %s\n", $_, /^(\d{1,3})${\($1 > 255 ? qr(^) : qr($))}/
         ? "matches" : "fails";
}

__DATA__
3
100
333
200


Use of uninitialized value in numeric gt (>) at XXX line 9, <DATA> lin
+e 1.
3 matches
100 matches
333 matches
200 fails
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