You should be able to call another script with system and it's output should be sent to the client as part of the page. Just make sure you flush your buffers before use system.
#!/usr/bin/perl
$| = 1;
print "Content-Type: text/html\n\n";
print "This is that start of the page<br>\n<br>\n<pre>\n";
system(qq(echo "This is from a system()"; env));
print "</pre>\nThis is back in the script<br>\n";