use Data::Dumper;
my %result; # Be careful here, I used a hash, not a hashref
{
  local $/ = ""; # make sure the change to $/'s value doesn't leek to the rest of the program
  
  while(<DATA>) # Read the lines at the end like a input file
  {
    if (/^([a-z][\w.]+)/i) # a line starting by a letter, followed by letters, numbers or a dot
    {
       my $alias = $1;
       my $sid = whoops!; # here some code to find the value of $sid
       
       $result{$sid} = $alias unless exists $result{$sid}; # keep the alias unless the sid is already known
    }
}
print Dumper \%result; # show result

__DATA__
DB1.UK, DB2.UK =
  (
    (ADDRESS = (PROTOCAL = TCP))
    (CONNECT_DATA = (SID = db1))
  )

DB1.EU, DB2.CH =
  (
    (ADDRESS = (PROTOCAL = TCP))
    (CONNECT_DATA = (SID = db1))
  )

DB3.UK =
  (
    (ADDRESS = (PROTOCAL = TCP))
    (CONNECT_DATA = (SID = db3))
  )

DB3.US =
  (
    (ADDRESS = (PROTOCAL = TCP))
    (CONNECT_DATA = (SID = db3))
  )

DB5.UK.US, DB5.US =
  (
    (ADDRESS = (PROTOCAL = TCP))
    (CONNECT_DATA = (SID = db5))
  )