#!/usr/bin/perl -w
#use strict;	# can optionally enable to show no "unstrict" constructs

##
##	this prog demonstrates 3 related problems and a 4th example
##  that may also be a problem; (all 4, likely same cause)
##  See descriptive text below after __END__
##

{ package F;
  our $count=-1;
  sub boom { ++$count; $count==0 ? "two\n": $count==1 ?"one\n":"unprint\n";}
}
{ package KA; our @ISA = qw(F);
  ##  PROBLEM #1 'can' works even when 'boom' can't be called
  ##  PROBLEM #3b ( -- call to boom fails 2nd time through)
  ##  called as valid method
  sub innocent{ if ($_[0]->can("boom")) {$_[0]->boom; }}        #ln 18
  #
  sub laytrap { 
    { 
    my @dummy=(\&boom);                                         #ln 22
      #  (see comments below about 4th "concern")
      #  my $boom_defined=$dummy[0];                            #ln 24
      #  print "boom defined = " . defined $boom_defined;       #ln 25 
      #  print "; boom exists = " . exists $boom_defined;       #ln 26
      #  print "\n";                                            #ln 27
    } "zero:trap laid\n" 
  }
}
package main;
  my $o = bless [],"KA";

  ##  These two calls (boom & innocent work...
  print $o->boom;   print $o->innocent;

  ##  lay trap to break boom & innocent
  print $o->laytrap;

  ##  Now same two calls, boom & innocent, will fail:
  ##  PROBLEM #2 -- can no longer call parent method 'boom'
  ##  (using eval to catch error)
  eval {if ($o->can("boom")) {print $o->boom}};                #ln 43
  $@ && print "$@";

  ##  PROBLEM #3 - within 'innocent', call to 'boom' fails
  ##  this call terminates program (fatal Undefined error)
  print $o->innocent;

__END__

The above prog demonstrates three (3) OO-call related problems.
Perl autovivifies "\&boom" into "@dummy" in "sub laytrap" (ln 22,
above). This *interferes* with OO calls through a blessed ref
later on in program execution.

The 4th "problem is the commented-out section above numbered lines
"24-26" (in comments).  _*If*_ you can autovivify functions or
methods, then "exists()" should work with function references.
Instead, if you uncomment lines 24-27, you will get an error:
"exists argument is not a HASH or ARRAY element at ./pbug1 line 26."

On the other hand, if you allow "exists" to work with coderefs, 
why not "simple vars"?