my %relays;
while (<>) {
    my ($id, $state) = split ' ';
    push @{ $relays{$id} }, $state;
}

for my $id (keys(%relays)) {
    print("$id @{ $relays{$id} }\n");
}
[download]

Thanks - that worked great! ..but it got me thinking. How could I do the same thing only instead of processing your code on a finished file if I could do the same thing as the file is being built? In other words, when code hits a relay that has already appeared with a state, the newly read state would be "pushed" along side of the previous one.

Just replace the line that writes to the file with push @{ $relays{$id} }, $state;

my %states;
# fill %states with data
my $serialized = join('/', map {"$_ $states{$_}"} keys %states);
print {$filehandle} $serialized;
[download]

currently (as the file appears) relay1 reset relay5 initiate relay3 set relay8 reset relay1 off relay5 running desired relay1 reset off relay5 initiate running hope this helps..