use warnings;
use strict;

while(<DATA>){
    print if /
                cat # word cat
                (?: # do not capture
                (?! # negative lookahead
                \s* # possible whitespace
                dog # the word dog
                )   # end negative lookahead
                )   # end do not capture group
            /x;

    # /cat(?:(?!\s*dog))/
}

__DATA__
my cat loves dogs
catdog
cat dog dog dog
dog cat dog
dog cat not a dog