Answer by Stéphane Chazelas for A standard tool to convert a byte-count into human KiB MiB etc; like du, ls1

There are a couple of perl modules on CPAN: Format::Human::Bytes and Number::Bytes::Human, the latter one being a bit more complete:

$ echo 100 1000 100000 100000000 |
  perl -M'Number::Bytes::Human format_bytes' -pe 's/\d{3,}/format_bytes($&)/ge'
100 1000 98K 96M

$ echo 100 1000 100000 100000000 |
  perl -M'Number::Bytes::Human format_bytes' -pe 's/\d{3,}/
   format_bytes($&,bs=>1000, round_style => 'round', precision => 2)/ge'
100 1.00k 100k 100M

And the reverse:

$ echo 100 1.00k 100K 100M 1Z |
  perl -M'Number::Bytes::Human parse_bytes' -pe '
    s/[\d.]+[kKMGTPEZY]/parse_bytes($&)/ge'
100 1024 102400 104857600 1.18059162071741e+21

NOTE: the function parse_bytes() was added in version 0.09 (2013-03-01)