Well, quick and dirty, and at the cost of a performance hit everywhere, you could overload numification etc. in UNIVERSAL to carp about it:

WarnOver.pm

package WarnOver;
use strict;
use warnings;

package UNIVERSAL;
use Scalar::Util 'refaddr';
use Carp;
use overload
    q{0+} => sub { 
        carp "numifying reference " . overload::StrVal($_[0]); 
        refaddr $_[0];
    },
    q{""} => sub { 
        carp "stringifying reference " . overload::StrVal($_[0]);
        overload::StrVal($_[0]); 
    },
    fallback => 1;
;
1;
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test_warnover.pl

use strict;
use warnings;
use lib ".";
use WarnOver;

my $foo = bless {}, "Foo";

$\="\n";
print $foo % 2;
print "$foo";
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Result

numifying reference Foo=HASH(0x225118) at checkwarn.pl line 10
0
stringifying reference Foo=HASH(0x225118) at checkwarn.pl line 11
Foo=HASH(0x225118)
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Update: This was just a quick thought exercise in evil overloading. It of course breaks horribly for object comparisions, as discussed above.

package Foo;
use WarnOver;
$foo = bless {};
$bar = bless {}; 
print "equal" if $foo == $bar; # two warnings!
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