Try adding this to your code:

print "<b>$_</b>: $ENV{$_}<br>\n" for sort keys %ENV;
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You should see the variables SCRIPT_TRANSLATED and SCRIPT_NAME, which contain the information you need.
SCRIPT_NAME is essentially the host absolute URL (minus the scheme, host, and port) for script, and SCRIPT_TRANSLATED is the corresponding physical file.
You can extract the path from these, and use that to prefix your "data/file.txt" before opening it.