If your going to go for the concatenated keys route, then it would be better to use a separator rather than null in the joins to prevent {AA=>1, B=>2} comparing equal to {A=>1, AB=>2}.

chr(28), the default value of $; is probably as good as any other. Usual caveats apply.

#! perl -slw
use strict;

sub cmp_hashes {local $"=$;;  #"
    "@{[ sort keys %{$_[0]} ]}" cmp "@{[ sort keys %{$_[1]} ]}"
}

my %hashA = ( A=>1, B=>2, C=>3, D=>4 );
my %hashB = ( A=>1, B=>2, C=>3, D=>4 );
my %hashC = ( A=>1, C=>3, D=>4 );
my %hashD = ( A=>1, AB=>2, C=>3, D=>4, E=>5 );
my %hashE = ( AA=>1, B=>2, C=>3, D=>4, E=>5 );

print "hashA cmp hashB =>", cmp_hashes \%hashA, \%hashB;
print "hashA cmp hashC =>", cmp_hashes \%hashA, \%hashC;
print "hashA cmp hashD =>", cmp_hashes \%hashA, \%hashD;
print "hashA cmp hashE =>", cmp_hashes \%hashA, \%hashE;
print "hashB cmp hashC =>", cmp_hashes \%hashB, \%hashC;
print "hashB cmp hashD =>", cmp_hashes \%hashB, \%hashD;
print "hashB cmp hashE =>", cmp_hashes \%hashB, \%hashE;
print "hashC cmp hashD =>", cmp_hashes \%hashC, \%hashD;
print "hashC cmp hashE =>", cmp_hashes \%hashC, \%hashE;
print "hashD cmp hashE =>", cmp_hashes \%hashD, \%hashE;

__END__
C:\test>244656
hashA cmp hashB =>0
hashA cmp hashC =>-1
hashA cmp hashD =>1
hashA cmp hashE =>-1
hashB cmp hashC =>-1
hashB cmp hashD =>1
hashB cmp hashE =>-1
hashC cmp hashD =>1
hashC cmp hashE =>-1
hashD cmp hashE =>-1
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