I believe that your msgs hash may be out of scope when you call msg. I tried your code and ran into the same problems. I rewrote it like this

my $msgs = {    '200'   =>  'OK',
                '400'   =>  'Bad Request',
                '403'   =>  'Forbidden',
                '404'   =>  'Not Found',
                '500'   =>  'Internal Sever Error',
            };

sub msg {
    print "HTTP/1.0 $_[0] $msgs->{$_[0]}\n";
}

if (1 == 1) {
    msg(400);
}
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And it seems to work fine.

On a side note you don't need double quotes around your hash vars, as they're not interpolated. Just something I learned and thought I'd pass on. It is supposedly a performance hit, although I've never tested it myself.