I take it that variables still hang around at the start of a new package even though they are no longer directly acessible ( I am supposing that putting { and } around the entire package would change this, at least the samples for Class::Std do it, and there would have to be a reason.)

No, source code files are already treated as if they were surrounded by { and }, and variables are normally freed at the end of executing the included file. If a sub in that file captures the variable, then its lifetime is extended, but only that sub can subsequently access the retained value. For example:

# Foo.pm
my $var = 1;
sub a {
    eval '$var' # sees an undefined value when later called
}
sub b {
    $var;
    eval '$var'; # sees 1 because b() has captured $var
}
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Note that the variable's lifetime is extended merely by the existence of b(), but even with the existence of b(), a() cannot see the variable.

Dave.