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sub IRS_chunky {
    my($buf, $leftover, @lines);
...
    
    close(FILE);
}

perl test_read.pl
Benchmark: timing 10 iterations of Chunky IRS, chunk, linebyline...
Chunky IRS: 41 wallclock secs (31.31 usr +  3.71 sys = 35.02 CPU)
     chunk: 40 wallclock secs (31.00 usr +  3.81 sys = 34.81 CPU)
linebyline: 27 wallclock secs (17.67 usr +  2.47 sys = 20.14 CPU)