根据 RFC3875 （第4.1节）中的CGI规范。 4.），GATEWAY_INTERFACE环境变量是检查您是否在CGI上下文中运行的权威性操作：

4.1.4。 GATEWAY_INTERFACE

GATEWAY_INTERFACE 变量必须设置为CGI
的方言被服务器用来与脚本通信。

I have a script that I wrote that can either be used on the command line or as a CGI script, and need to determine how the script was called so I can output a content-type header for web requests (and maybe some anti-cache headers too). My first thought is to check for the existance of http environment variables:

my $js = build_javascript();

if ( exists $ENV{HTTP_HOST} ) {
   print "Content-type: text/javascript\n\n";
}
print $js;

Is there a better way?

According to the CGI specification in RFC3875 (section 4.1.4.), the GATEWAY_INTERFACE environment variable would be the authoritative thing to check whether you are running in a CGI context:

4.1.4. GATEWAY_INTERFACE

The GATEWAY_INTERFACE variable MUST be set to the dialect of CGI being used by the server to communicate with the script.