use strict;
use warnings;

my @strings = qw(exception:tex exception:mex asdf tex:exception:mex);

for ( my $i; $i < 500; $i++ ) {
   my @unfiltered = @strings;
   my @filtered = grep { /exception:(?!tex)/} @unfiltered; };
   my @unfiltered = @strings;
   my @filtered1 = grep { /exception/ && !/tex/ } @unfiltered; };
   my @unfiltered = @strings;
   my @filtered2 = grep { /exception:/g && !/\Gtex/ } @unfiltered; };
}