My reading of perlvar is that you can't set $! to a particular string. You can set it to a particular number so that it will stringify to a particular error message, but that's it.

use strict;
use warnings;

$! = 'foo';
print "bang: $!\n";
$! = 3;
print "bang: $!\n";
__END__
Argument "foo" isn't numeric in scalar assignment at perlmonks.pl line
+ 8.
bang:
bang: No such process
[download]

I'd suggest you use a package variable to pass your error messages or throw full blown exceptions (see Exception::Class) with as much detail as you like.