$ perl -MData::Dumper -de1
  DB<1>  $Data::Dumper::Purity=1;

  DB<2> $d = \do { my $a = 'blah blah' };

  DB<3> x $d
0  SCALAR(0x830501c)
   -> 'blah blah'

  DB<4> s/b/B/g for $$d;

  DB<5> x $d
0  SCALAR(0x830501c)
   -> 'Blah Blah'

  DB<6> p Dumper $d
$VAR1 = \'Blah Blah';

  DB<7> x eval Dumper $d
0  SCALAR(0x839c84c)
   -> 'Blah Blah'

  DB<8> s/B/X/g for (${eval Dumper $d});
Modification of a read-only value attempted at (eval 11)[/usr/share/perl/5.6.1/perl5db.pl:1521] line 2.