Roy Johnson,
Thanks - your suggestion lead to getting rid of the %common/master hash all together

my %slave = map {chomp; $_ => undef} <SLAVE>;
while ( <MASTER> ) {
    chomp;
    exists $slave{$_} ? delete $slave{$_} : print "$_ exists on master
+ but not slave\n";
}
print "$_ exists on slave but not master\n" for keys %slave;
[download]

Cheers - L~R