while ( <> ) {
    if ( /\(\+Header1\)/ .. /\(-Header1\)/ ) {
        # you're in part one
    }
    if ( /\(\+Header2\)/ .. /\(-Header2\)/ ) {
        # you're in part two
    }
    if ( /\(\+Header3\)/ .. /\(-Header3\)/ ) {
        # you're in part three
    }
}