That is not doing what you think. Consider:

use strict;
use warnings;

while (<DATA>) {
    print "matched $_" if /^[^cat)(?:]/;
}

__DATA__
cat dog
dog cat
aardvark ant
catnip dogbone
frog cat dog
mouse dog cat
cat dog elephant
can of worms
[download]

Prints:

matched dog cat
matched frog cat dog
matched mouse dog cat
[download]

The following code excludes beginning cats and insists that dogs end:

use strict;
use warnings;

while (<DATA>) {
    print "matched $_" if /^(?!cat\b).*(?=\bdog$)/;
}

__DATA__
cat dog
dog cat
catnip dogbone
aardvark ant
frog cat dog
can of worms
[download]

Prints:

matched frog cat dog
[download]