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sub unroll {
    my @x = $_[0] =~ /(?:^|@)((?:##|#@|[^#@])*)/g;
...
        $_ =~ s/#@/@/g;
    } @x
}

unroll("#@##@###@####@#####@")

    '@#',
    '#@##',
    '##@',

unroll("@@#@##@###@####@#####@")

    '',
    '',
    '@#',
    '#@##',
    '##@'

sub my_escape {
    my $x = shift;
...
sub my_join {
    join('@',@_)
}

my_join(map{my_escape($_)}('','','@#','#@##','##@',))

'@@#@##@###@####@#####@'