What a waste. This will slow down the program by so much and it'll use up so much more memory than needed. You could simply use 
use strict;
use warnings;

my $bad = 0;
while (<>) {
   if (!/^[actg]+$/i) {
      ++$bad;
      last;
   }
}

print $bad ? "BAD\n" : "OK\n";

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In reply to Re^2: =~ matches non-existent symbols by ikegami
in thread =~ matches non-existent symbols by Anonymous Monk

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