% perl -MData::Dumper -MMath::GMP -wle '
  $Data::Dumper::Freezer = "zify";
  local *UNIVERSAL::zify = sub { warn "zify $_[0]"; $_[0]->isa("Math::GMP") ? "z$_[0]" : $_[0] };
  my $x = [ Math::GMP->new(1) ];
  print "x[0]=$x->[0]";
  print Dumper($x);
  print "x[0]=$x->[0]";
'
Name "UNIVERSAL::zify" used only once: possible typo at -e line 3.
x[0]=1
zify 1 at -e line 3.
$VAR1 = [
          bless( do{\(my $o = 40040032)}, 'Math::GMP' )
        ];

x[0]=1

##</code><code>##

% perl -MData::Dumper -MMath::GMP -wle '
  $Data::Dumper::Freezer = "zify";
  local *UNIVERSAL::zify = sub { warn "zify $_[0]"; $_[0]->isa("Math::GMP") ? "z$_[0]" : $_[0] };
  my $x = [ Math::GMP->new(1) ];
  print "x[0]=$x->[0]";
  print Dumper($x);
  print "x[0]=$x->[0]";
'
Name "UNIVERSAL::zify" used only once: possible typo at -e line 1.
x[0]=1
zify 1 at -e line 1.
$VAR1 = [
          \undef
        ];

x[0]=z1