Use the debugger... put a breakpoint on the line before the call $DB::single = 1;:

$DB::single = 1;
$des->all();
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Then run it from the command line with the -d param: perl -d script.pl. You'll be dropped into the debugger. Simply type c to 'continue' all the way to your breakpoint, then s to 'step' into the next line of code which should give you some insight:

main::(test.pl:6):    my $des = Devel::Examine::Subs->new({file => 'sh
+ift.pl'});
  DB<1> c
main::(test.pl:9):    $des->all();
  DB<1> s
Devel::Examine::Subs::all(/usr/lib/perl5/site_perl/5.22.0/Devel/Examin
+e/Subs.pm:382):
382:        my $self    = shift;
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-stevieb