If the goal is a module that can be used in multiple instances though then a package global is a horrible interface to begin with. How about something like this?

use Exporter::Tidy default => [ qw(transaction select_handle commit ro
+llback) ];

sub select_handle {
    require Carp;
    Carp::croak("Can't select handle outside transaction");
}

sub _cant_change {
    require Carp;
    Carp::croak("Can't change handle during transaction");
}

sub transaction (&) {
    my ($block) = @_;
    my ($caller, $handle);

    my $caller = caller();

    local *{"${caller}::select_handle"} = sub {
        $handle = shift;
        *{"${caller}::select_handle"} = \&_cant_change;
    };

    # ...
}
[download]

To use it you then have to say

transaction {
    select_handle($dhb);
    # ...
};
[download]

which allows to use the module for as many handles even within the same package as you like. You can even nest transactions with no adverse effects.

Makeshifts last the longest.

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