I found this interesting so I played with it briefly...

# main.pl
my @source = @{ $main::{ '_<' . __FILE__ } };
shift @source; # get rid of sub DB::DB{};
print for @source;
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PERL5DB="sub DB::DB{}" perl -d main.pl
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Output:

my @source = @{ $main::{ '_<' . __FILE__ } };
shift @source; # get rid of sub DB::DB{};
print for @source;
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Update: I had found @{$main::{'_<'.$filename}} yesterday but didn't have time to play with it.

Update 2: Added shift.