After reading in LStein's book, your use of $r->subprocess_env seems to be the wrong approach. "Changes made to the environment table only persist for the length of the request. The table is cleared out and reinitialized at the beginning of every transaction." (pg.455)

On pg. 437, it is noted that changing the path_info() requires syncing the uri() to the update. As in:

my $path_info = $r->path_info;
my $uri = $r->uri;
my $orig_uri = substr $uri, 0 length($uri) - length($path_info);
$r->path_info($path_info . $step);
$r->uri($orig_uri . $r->path_info);
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