You probably sent the HTTP header via print( ):
print "Content-type: text/html\n\n";
If this is the case, you must make sure that you have:
PerlSendHeader On
in the configuration part of the <Location>
section:
<Location /perl>
...
PerlSendHeader On
</Location>
This adds a little overhead to the output generation, because when
this configuration is enabled, mod_perl will parse the output and try
to find where the header information ends so it can be converted into
a proper HTTP header. It is meant only for mod_cgi emulation with
regard to HTTP headers.