I am assuming there is only 1 statement per file? You are just looking for more than one control statement? ie 2 selects, two deletes or one of each? I would most likely take a slightly different approach to most, I am sure you will get a dozen replies on the benefits of using maps for this, but a simple method could be something like this:

open(INFILE, "filename.txt");

my %hash;

while(<INFILE>)
{
   chomp;
   foreach my $e (split / /, $_) {
      defined($hash{uc($e)}) ? $hash{uc($e)} = 1 :              
                               $hash{uc($e)} += 1;
   }
}

if ( ($hash{'SELECT'} > 1) || ($hash{'DELETE'} > 1) )
{
   print "There is a nested SQL in this file\n";
}
elsif ( ($hash{'SELECT'} >= 1) && ($hash{'DELETE'} >= 1) )
{
   print "There is a nested SQL in this file\n";
}
else
{
   print "File is fine\n";
}
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