package Exception;
sub TIESCALAR { bless {}, $_[0] }
sub STORE { print "Changing the value from '$@' to '$_[1]' (not really)\n" }
sub FETCH { print "$@\n";42 }

package main;
my $t = tie $@, 'Exception' or die "Unable to do what you want\n";
my ($foo, $bar) = (0, 0);
$@ = "hi";
eval { $foo = 42 / $bar };
print "The value is : $@\n";
$t->STORE("taking a look");
print "Value is still: $@, though\n";
$@ = 'blah';
print "Now the value is : $@\n";
__END__
Changing the value from 'hi' to 'hi' (not really)
The value is : Illegal division by zero at - line 10.

Changing the value from 'Illegal division by zero at - line 10.
' to 'taking a look' (not really)
Value is still: Illegal division by zero at - line 10.
, though
Changing the value from 'blah' to 'blah' (not really)
blah
Now the value is : 42