Either you don't send a content-type, or you don't send it as the first output. Does the following code work?

#! /usr/bin/perl
use warnings;
use strict;
use CGI;
use CGI::Carp qw(fatalsToBrowser warningsToBrowser);

my $cgi = CGI->new();
print $cgi->header(), $cgi->start_html('Testpage');
print $cgi->h1('Testpage');
print $cgi->end_html();
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