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;