SET @save_sqlf=@@global.slow_query_log_file;

--replace_result $MYSQL_TMP_DIR ...
eval SET GLOBAL slow_query_log_file= '$MYSQL_TMP_DIR/my_slow.log';

SET timestamp=10;
SELECT unix_timestamp(), sleep(2);

let SLOW_LOG= `SELECT @@global.slow_query_log_file`;

--perl
   use strict;

   my $file= $ENV{'SLOW_LOG'} or die("slow log not set");
   my $result=0;

   open(FILE, "$file") or die("Unable to open $file: $!");
   while (<FILE>) {
     my $line = $_;
     $result++ if ($line =~ /SET timestamp=10;/);
   }
   close(FILE);

   if($result != 1) {
     print "[ FAIL ] timestamp not found\n";
   }
   else {
     print "[ PASS ] timestamp found\n";
   }

EOF

SET @@global.slow_query_log_file=@save_sqlf;

--remove_file $MYSQL_TMP_DIR/my_slow.log