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.

In reply to Re: Negative Look-behind Assertion by tadman
in thread stupid question about regexp - not matching a word by Anonymous Monk

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