I am unclear about the $munged() bit of code in your description. &$munged() would work, were $munged a code reference. As for the effeciency of your approach, i would run it all through Benchmark, for any other opinions would be but opinions. It maybe quicker to do something like 
if ($uclc eq 'lc') {
  ... for loop with lc
} elsif ($uclc eq 'uc') {
  ... for loop with uc
} else {
    die "invalid arg";
}

[download]
it may be slightly fatser, but check benchmark
Thus spake the Master Programmer:
"When you have learned to snatch the error code from the trap frame, it will be time for you to leave."
-- The Tao of Programming

In reply to Re: More efficient munging if infile very large by MZSanford
in thread More efficient munging if infile very large by ybiC

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