Like I already said, my server provides the document root in $ENV{DOCUMENT_ROOT}. Check your server's documentation or just dump the environment variables

#!/usr/bin/perl
use strict;
use warnings;
use List::Util qw( max );
print("Content-Type: text/plain\n\n");
my $max_size = max map length, keys %ENV;
my $format = "%-" . ($max_size+1) . "s %s\n";
for (sort keys %ENV) {
   printf($format, "$_:", $ENV{$_});
}
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