Option A: Remove the word from your sentence:

s/\babc\s+//;

Option B: Print only lines without "abc" in them:

print unless /\babc\b/;

Note that the use of \b indicates that there should be a "word-boundary" there. This prevents accidental matches of things like "abcd" or "cbabc".

Option C: Check for something that isn't "abc":

print if /(\w+) nevermind/ && $1 ne "abc";

You could get fancy with look-behind assertions or embedded code, if you desire. See: perlre

print if /(?<!abc) nevermind/;

I think this last one is what you were trying for.