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while (<>) {
    if (/^#ERRORS/ .. /^CELLS/) {
...
        ...; # other processing
    }
}

if    (/match1/ .. /match2/) { doStage1(); }
elsif (.. /match3/)          { doStage2(); }
elsif (.. /match4/)          { doStage3(); }
else                         { doOtherProcessing(); }