And here's what you get if you comment out your signal handler.

NOTE: NO WARNINGS from my original entry.

I hade to comment out your implementation of tadman's solution because if wouldn't compile.

I think the problem lies in your signal handler rather than my code. I specifically disable the warnings where I accept they will occur using the standard Perl mechanism of local $^W=0;

Your signal handler converts a SIG_WARN into a SIG_INT (die) which kind of belies the reasons for Perl distinguishing between them.

If you change the rules of Formula 1 to have a mandatory water section and then enter in a D.U.K.W., you'll likely win every race.

use warnings;
use strict;
use Carp;
#local $SIG{__WARN__}=sub {die("\t@{[(caller(1))[3]]}() generated warn
+ings!\n")};

my $foo = {
  aa => { O => 'zz',  C => 'yy'  }, #
  bb => { O => 'xx',  C => 'xx'  },
  cc => { O => undef, C => 'ww'  }, #
  dd => { O => 'vv',  C => undef }, #
  ee => { O => undef, C => undef },
  ff => {       C => undef },
  gg => { O => 0,   C => 0   },
  hh => { O => undef, C => 0   }, #
  #ii => { O => undef       }, # commented out because of rules .
  jj => { O => 0,   C => ' '  }, #
  kk => { O => ' ',  C => 0   }, #
  ll => { O => undef, C => ' '  }, #
  mm => { O => ' ',  C => undef }, #
};

sub BUK{local($^W,$a)=(0,pop);grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a
+->{$_}{C}}keys%$a}
sub snw{no warnings qw(uninitialized);my$h=pop;grep{$h->{$_}{O}ne$h->{
+$_}{C}}keys %$h}
sub sg_{my$h=pop;grep{!(defined($h->{$_}{O}&&$h->{$_}{C})&&$h->{$_}{O}
+eq$h->{$_}{C}||!defined($h->{$_}{O}|| $h->{$_}{C}))}%$h}
sub eg_{my$h=pop;grep{!(($h->{$_}{O}||$;)eq($h->{$_}{C}||$;))}%$h}
sub ddf{$a=pop;grep{$b=$$a{$_};defined$$b{O}!=defined$$b{C}||defined$$
+b{O}&&$$b{O}ne$$b{C}}keys%$a}#98
sub dex{$a=pop;grep{$b=$$a{$_};exists$$b{O}!=exists$$b{C}||defined$$b{
+O}!=defined$$b{C}|| defined$$b{O}&&$$b{O}ne$$b{C}}keys%$a}#126
#sub tad{my($h)=@_;grep{!exists($h->{$_}->{O})||(defined($h->{$_}->{O}
+)?(defined($h->{$_}->{C})?($h->{$_}- >{O}ne$h->{$_}->{C}):1):defined(
+$h->{$_}->{C}))}keys%$h}#162
sub ukg{local$^W=0;my$a=pop;grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a->
+{$_}{C}}keys%$a} #85
sub ukf{local($^W,$a)=(0,pop);grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a
+->{$_}{C}}keys%$a} #87
sub ukh{$^W=0;$a=pop;grep{!exists$a->{$_}{O}or$a->{$_}{O}ne$a->{$_}{C}
+}keys%$a}

print "    Required: aa cc dd ff hh jj kk ll mm\n";
print "------------------------------------------\n";
eval { print "  tadman golf: @{[sort {$a cmp $b} tad($foo)]}\n"; };
print $@ if $@;

eval { print "BrowserUK golf: @{[sort {$a cmp $b} BUK($foo)]}\n"; };
print $@ if $@;

eval { print "sauoq No warns: @{[sort {$a cmp $b} snw($foo)]}\n"; };
print $@ if $@;

eval { print "   Sauoq golf: @{[sort {$a cmp $b} sg_($foo)]}\n"; };
print $@ if $@;

eval { print "  Elusion golf: @{[sort {$a cmp $b} eg_($foo)]}\n"; };
print $@ if $@;

eval { print " Demerphq defd: @{[sort {$a cmp $b} ddf($foo)]}\n"; };
print $@ if $@;

eval { print "Demerphq exist: @{[sort {$a cmp $b} dex($foo)]}\n"; };
print $@ if $@;

eval { print " browserUk f(): @{[sort {$a cmp $b} ukf($foo)]}\n"; };
print $@ if $@;

eval { print " browserUk g(): @{[sort {$a cmp $b} ukg($foo)]}\n"; };
print $@ if $@;

eval { print " browserUk h(): @{[sort {$a cmp $b} ukh($foo)]}\n"; };
print $@ if $@;

__END__
21:07:50.19 C:\test>201598
    Required: aa cc dd ff hh jj kk ll mm
------------------------------------------
Undefined subroutine &main::tad called at C:\test\201598.pl line 35.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
Use of uninitialized value in string ne at C:\test\201598.pl line 22.
BrowserUK golf: aa cc dd ff hh jj kk ll mm
sauoq No warns: aa cc dd hh jj kk ll mm
   Sauoq golf: aa cc dd hh jj kk ll mm
  Elusion golf: aa cc dd jj kk ll mm
 Demerphq defd: aa cc dd hh jj kk ll mm
Demerphq exist: aa cc dd ff hh jj kk ll mm
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
Use of uninitialized value in string ne at C:\test\201598.pl line 30.
 browserUk f(): HASH(0x1bd5534) HASH(0x1bd5594) HASH(0x1bd55f4) HASH(0
+x1bd5654) HASH(0x1bd569c) HASH(0x1bd56fc) HASH(0x1bd575c) HASH
(0x1bdf0dc) HASH(0x1bdf1c0) HASH(0x1d1f664) HASH(0x1d1f6ac) HASH(0x1d1
+f6f4) HASH(0x1d1f73c) HASH(0x1d1f784) HASH(0x1d1f7cc) HASH(0x1
d1f814) HASH(0x1d231cc) HASH(0x1d23214) HASH(0x1d2325c) HASH(0x1d2328c
+) HASH(0x1d232d4) HASH(0x1d241e8) HASH(0x1d243c8) HASH(0x1d244
c4) aa cc dd ff hh jj kk ll mm
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
Use of uninitialized value in string ne at C:\test\201598.pl line 29.
 browserUk g(): HASH(0x1bd5534) HASH(0x1bd5594) HASH(0x1bd55f4) HASH(0
+x1bd5654) HASH(0x1bd569c) HASH(0x1bd56fc) HASH(0x1bd575c) HASH
(0x1bdf0dc) HASH(0x1bdf1c0) HASH(0x1d1f664) HASH(0x1d1f6ac) HASH(0x1d1
+f6f4) HASH(0x1d1f73c) HASH(0x1d1f784) HASH(0x1d1f7cc) HASH(0x1
d1f814) HASH(0x1d231cc) HASH(0x1d23214) HASH(0x1d2325c) HASH(0x1d2328c
+) HASH(0x1d232d4) HASH(0x1d241e8) HASH(0x1d243c8) HASH(0x1d244
c4) aa cc dd ff hh jj kk ll mm
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
Use of uninitialized value in string ne at C:\test\201598.pl line 31.
 browserUk h(): HASH(0x1bd5534) HASH(0x1bd5594) HASH(0x1bd55f4) HASH(0
+x1bd5654) HASH(0x1bd569c) HASH(0x1bd56fc) HASH(0x1bd575c) HASH
(0x1bdf0dc) HASH(0x1bdf1c0) HASH(0x1d1f664) HASH(0x1d1f6ac) HASH(0x1d1
+f6f4) HASH(0x1d1f73c) HASH(0x1d1f784) HASH(0x1d1f7cc) HASH(0x1
d1f814) HASH(0x1d231cc) HASH(0x1d23214) HASH(0x1d2325c) HASH(0x1d2328c
+) HASH(0x1d232d4) HASH(0x1d241e8) HASH(0x1d243c8) HASH(0x1d244
c4) aa cc dd ff hh jj kk ll mm
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