You have to be careful here, because you are doing exactly the same thing. The foreach statement is also accessing <FB> in array context and is reading the whole file into an anonymous array and then iterating over it. You have to use a while loop for this to work correctly.

Try out the following bit of code to test this:

use Benchmark;

timethese(1, {
  'Trial1 While' => sub {
      open (FILE, "file2") or die "Can't open file:  $!\n";
      while (<FILE>) {
          last; # read one line and exit
      }
      close FILE;
  },
  'Trial2 Foreach' => sub {
      open (FILE, "file1") or die "Can't open file:  $!\n";
      foreach my $line (<FILE>) {
          last; # read one line and exit
      }
      close FILE;
  },
});
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Make sure that file1 and file2 are identical (I used two files so that we know there is no caching going on), and that they are large text files. I got the following results with 2 50Meg files:

Benchmark: timing 1 iterations of Trial1 While, Trial2 Foreach...
Trial1 While:  0 wallclock secs ( 0.00 usr +  0.00 sys =  0.00 CPU)
Trial2 Foreach: 17 wallclock secs (10.77 usr +  1.56 sys = 12.33 CPU) 
+@  0.08/s (n=1)
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