The problem is it's using $` which incurs a huge performance penalty.

Here's another OWTDI:

$_ = join "", map { $_ . " " x (8 - length() % 8) } split /\t/, $_, -1; Update: blakem points out that this code is broken. It will pad all parts of the string to a multiple of 8, including the last one. Getting it not to is disappointingly awkward.. 
$_ = join "", do {
    my @bits = split /\t/, $_, -1;
    (map { $_ . " " x (8 - length() % 8) } @bits[0..$#bits-1]), $bits[
+-1]
};

[download]
(Update is untested.)

Makeshifts last the longest.

In reply to Re^2: How to replace Tab with spaces not altering postion by Aristotle
in thread How to replace Tab with spaces not altering postion by juo

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