#!/usr/bin/perl -w
use strict;

my $logfile = "/buildarea/logfile.log";
my $buildmsg = "";

open(FH,"$logifle") or die "Can't open $logfile";

$_=<FH>;

if(/BUILD SUCCESSFUL/i){
   $buildmsg = "BUILD SUCCESSFUL";
   close FH;
}
else{
   $buildmsg = $_;

   while(<FH>){
      $buildmsg .= $_;
   }
close FH;
}
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The above is untested, and is assuming that if a successful build occurs
that the message "SUCCESSFUL BUILD" will be the only thing inside the logfile

UPDATE:Thanks to AgentM for pointing out that by inserting last in the while loop, the script will end much more quickly.
UPDATE 2:Thanks also to chipmunk for noticing that the loop would have worked incorrectly by not even reading in the 1st line of the file. I made changes to the way the file is read, and it should work now.

TStanley
In the end, there can be only one!