If I understand your task, you could just do it this way:

#!/usr/bin/perl

use strict;
use warnings;

while (<>) {
    my @ctrl = ( /([[:ctrl:]])/g );
    if ( @ctrl ) {
        my $cstr = join ",", map { sprintf "x%2.2x",ord() } @ctrl;
        s/([[:ctrl:]])/sprintf("||%x2.2x||",$1)/eg;
        printf( "line %d: ctrl-chars %s in <<%s>>\n",
                $., $cstr, $_ );
    }
}
[download]

Run it like this:

script_name  memo.txt > memo.errs
[download]

It'll behave a little differently from your version: if a single line contains two or more control characters, this will show all of them (e.g. "x13,x01,x11"), instead of showing just the first one.

That could also be done as a one-liner (using "perl -lne"), but then you might have shell quoting issues that I don't want to get into...

(UPDATE: Added a line of code inside the "if(@ctrl)" block, to mark the locations in the data line where the control characters occurred, as per OP's stated intention.)