I'd go for something like this (untested):

  my %abbr = (
    'k' => 1e3,
    'kb' => 1 << 10,
    'g' => 1e9,
    'gb' => 1 << 30,
  );
  my $match = join '|', keys %abbr;
  ... and later
  $text =~ s/(\d+)($match)\b/$1 * $abbr{lc $2}/gie;
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Note that since the pattern requires a word boundary following the abbreviation, it doesn't matter that the alternatives might try to match "k" before "kb". (Otherwise it would have needed a way to avoid that, eg by sorting the keys in reverse ASCII order.)

If you need to cope with more than just integers, it'd probably be worth looking in Regexp::Common for a way to match the numbers you want.

Hugo