Simpler: 
perl -nle"print /(DOG)/ ? $1 : 'Not found'"

[download]
Or if you wanted to keep the -p: 
perl -ple"$_ = /(DOG)/ ? $1 : 'Not found'"

[download]

In reply to Re^2: Replacing a search/not found pattern with a word by ikegami
in thread Replacing a search/not found pattern with a word by pinpe

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