Is there a way to get the regex to fail in the cases where true is not printed?

This looks simple at the first moment (maybe I didn't correctly understand the task you intended). You could generate a regular expression 'on thy fly' depending on the captured value.

$_ = 254;

my $r = qr{ ^                    # regex bound to start of line
            ((?>\d+))            # what to capture, don't backtrack: (
+?> )
            (??{
                $1<255 && $1>233 # what is looked for
                ? ''             # if yes, let the regex succeed
                : '(?!)'         # if no, let the regex bail
            }) 
          }x;

print "true ($1)" if /$r/;
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You can't "modify" the regex outcome from within a simple code assertion ?{}, you'll need to use the dynamic regex assertion ??{} for that.

Regards

mwa