Thanks BrowserUk but in your example @unfiltered is empty.

use strict;
use warnings;
use Benchmark;

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

Benchmark::cmpthese( -5, {
    'one'   => q[ my @filtered = grep { /exception:(?!tex)/} @strings;
+ ],
    'two'   => q[ my @filtered = grep { /exception/ && !/tex/ } @strin
+gs; ],
    'three' => q[ my @filtered = grep { /exception:/g && !/\Gtex/ } @s
+trings; ],
});
__END__
          Rate   one   two three
one   127000/s    --  -21%  -22%
two   161678/s   27%    --   -1%
three 163855/s   29%    1%    --
[download]

This is still quite a bit different to the performance with the initializations included:

use strict;
use warnings;
use Benchmark;

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

Benchmark::cmpthese( -5, {
    'one'   => q[ my @unfiltered = @strings; my @filtered = grep { /ex
+ception:(?!tex)/} @unfiltered; ],
    'two'   => q[ my @unfiltered = @strings; my @filtered = grep { /ex
+ception/ && !/tex/ } @unfiltered; ],
    'three' => q[ my @unfiltered = @strings; my @filtered = grep { /ex
+ception:/g && !/\Gtex/ } @unfiltered; ],
});
__END__
          Rate three   one   two
three  81985/s    --  -14%  -24%
one    95390/s   16%    --  -12%
two   108473/s   32%   14%    --
[download]

so this illustrates that even simple initialization can have significant impact on results, and I imagine cases where it might be much more (e.g. setting up initial conditions for a database query, where the database itself must be initialized each time, and the connection to it, and caches flushed, etc.).