collect '//server/corion/images/';
    output '/home/corion/imagestream/output';
    size 160;
    size 640;
    exclude_tag 'private';
    prefer '.svg' => '.jpg';
    prefer '.svg' => '.png';
    prefer '.cr2' => '.jpg';

##</code><code>##

    sub collect($) {
        push @image_directories, @_;
    };
    sub size($) {
        push @image_sizes, @_;
    };
    sub prefer($$) {
        push @prefer_format, [@_];
    };
    ...

##</code><code>##

    my $spec = {
      collect => {
        name => 'collect',
        arg_count => 1,
        desc => 'A directory from which to collect images',
      },
      prefer => {
        name => 'prefer',
        arg_count => 2,
        desc => 'Prefer this image format over files with the same name in other formats',
      },
    # ...
    };

##</code><code>##

    my %handler;
    for my $item (values %$spec) {
        my $n = $item->{name};
        my $fetch;
        $result->{$n} = [];
        if (0 == $item->{arg_count}) {
            $fetch = sub() { push @{ $result->{$n}}, 1; };
        } elsif (1 == $item->{arg_count}) {
            $fetch = sub($) { push @{ $result->{$n} }, @_; };
        } elsif (2 == $item->{arg_count}) {
            $fetch = sub($$) { push @{ $result->{$n} }, [@_]; };
        } else {
            $fetch = sub($;) { push @{ $result->{$n}}, [@_]; };
        };
        $handler{$n} = $fetch;
    };

    # Install the subroutines
    for (keys %handler) {
        no strict 'refs';
        *{$_} = $handler{ $_ };
    };
    
    # "execute" the config:
    my $cfg_str = join ";\n", 
                     "package " . __PACKAGE__,
                     "#line $config_source#1",
                     "$config_data",
                     "1";
    #warn $cfg_str;
        
    my $ok = eval $cfg_str;
    my $err = $@;

    if ($ok) {
        return $result
    } else {
        warn $err;
        return
    };

##</code><code>##

use Scope::Guard qw(guard);
{ 
    my @guards;
    for (keys %handler) {
        # Save the old state:
        my($k,$p) = ($_,\&$_);
        push @guards, guard { 
            # restore the old state
            *$k = $p;
        };
        # Install our new handler
        *$_ = \&$handler{$_};
    }
    
    # "execute" the config
    # ...
}
# Scope::Guard executes our guard blocks here

##</code><code>##

    local @{My::App}{keys %handler} = values %handler;
    # Can't modify glob elem in local at ...

##</code><code>##

    for my $n (keys %handler) {
        local *$n = $handler{ $n };
    }
    # no handlers defined here anymore

##</code><code>##

    my ($ok,$err);
    my $cfg_str = join ";\n", 
                     "package " . __PACKAGE__,
                     "#line $config_source#1",
                     "$config_data",
                     "1";
    my @handlers = keys %handler;
    {
        no strict 'refs';
        
        # We don't want to introduce another scope, as that will
        # negate the effect of the local:
        NEXT:
            my $n = shift @handlers;
            local *{$n} = $handler{ $n };
            goto NEXT
                if @handlers;
                
        $ok = eval $cfg_str;
        $err = $@;
    }

    if ($ok) {
        return $result
    } else {
        warn $err;
        return
    };

##</code><code>##

    ...
    {
        my $guard = local_install_subs \%handler;
        $ok = eval $cfg_str;
        $err = $@;
    }
    ...