/usr/share/apt-cacher/apt-cacher-cgi.pl is in apt-cacher 1.7.13+deb9u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/usr/bin/perl
# cgi.pl - CGI to provide a local cache for debian packages and
# release files and .deb files. Actually just a wrapper to set CGI mode flag
# for the real script.
$ENV{CGI_MODE}=1;
# identify as CGI and run the actual script
require "/usr/share/apt-cacher/apt-cacher";
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