I took your idea, following ikegami's advice, and wrote a test.

#!/usr/bin/perl -l

use strict;
use CGI;
use Test::SubCalls;
use Test::More tests => 2;

BEGIN {
    my $debug = 1;
    if ($debug) { 
        require warnings;
        import warnings;
    }
}
print "Using warnings";
sub_track( 'CGI::new' );

my $q1 = CGI->new;
my $q2 = CGI->new;

sub_calls( 'CGI::new', 2, "Running" );
sub_calls( 'CGI::new', 2, "Ran" );
done_testing(2);
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