use Mail::Box::Manager;
use Mail::Box::Tie::HASH;

use strict;
use warnings;

die "usage: $0 copy orig\n" unless @ARGV == 2;

my ($copy_file, $orig_file) = @ARGV;

my $mgr = Mail::Box::Manager->new;

my $copy_folder = $mgr->open(access => 'rw', folder => $copy_file);
my $orig_folder = $mgr->open(access => 'r',  folder => $orig_file);

tie my %copy, 'Mail::Box::Tie::HASH', $copy_folder;
tie my %orig, 'Mail::Box::Tie::HASH', $orig_folder;

while (my $msgid = each %orig) {
    $copy{$msgid}->delete;
}