That can also be written using positive logic, as follows: 
while (<DATA>) {
    print if m/foo (\d+) bar (\d+)/ && $1 == $2;
}

__DATA__
foo 1000 bar 1000
foo 1000 bar 500

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In reply to Re^2: regex search valid only if registers n and n+1 are equal? by ikegami
in thread regex search valid only if registers n and n+1 are equal? by Voronich

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