Hmm. And just when I thought it was safe to go back into the water, I added one more thing into the mix. Both subroutines contain: while ( <FH> ) { .. and below each such line, I added: next if $_ !~ /mapi/io; ("MAPI" is the string being tested for.)

It's the crudest possible test, of course. But why bother with further processing if the string doesn't appear anywhere in the current line?

With that statement added, there was a considerable difference in the benchmarked results:

Benchmark: timing 30 iterations of REGEXP, SPLIT...
    REGEXP: 14 wallclock secs (14.35 usr +  0.19 sys = 14.54 CPU)
     SPLIT: 17 wallclock secs (16.01 usr +  0.28 sys = 16.29 CPU)
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I should have read my own pep-talk about trying stuff...