[Boston.pm] more specific regex error messages?

I have a rather complex regex that I'm throwing text at.

if($str=~m/\Athing1\:thing2\,thing3\@(\w+)\?(\d+)\Z/){
    # process str
} else {
    die "bad str 'str' ";
}

Is there a way to get better built in reporting with regexps?
with the above, if the string doesn't match, I get "bad input".

What I'd rather get is something like "expecting \:thing2"

if need be, I'll break it into a series of nested ifs.
but that would be exceedingly ugly.

The pattern does not require backtracking.
I've already checked that I'm in a certain location and only one kind of 
input is expected.

and again, I can't load external modules at work.

suggestions?
Greg