I guess we can start by getting rid of the useless variable and shorten the silly pattern.

my $split = join '/', split //, $id;
substr($split, -2, 2, '');
my $url = "$base_url/$split/$id/$id-h/$id-h.htm";
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Two lines to calculate and one line to assemble. I don't think length is really a problem here. We're dealing with readability issues if we try to shorten it any more. These are just too complicated:

substr( ( my $split = join '/', split //, $id ), -2, 2, '');
my $url = "$base_url/$split/$id/$id-h/$id-h.htm";
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( my $url = join '/', split //, $id ) =~
    s{(.*)/}{$base_url/$1/$id/$id-h/$id-h.htm}s;
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I'm partial to this *longer* version:

my $url = join('/', 
    $base_url,
    $id =~ /(.)(?=.)/sg,
    $id,
    "$id-h",
    "$id-h.htm"
);
[download]

The flow is very simple, so it's easy to understand.