Something like this perhaps:

#!/usr/local/bin/perl

use strict;
use warnings;

my %conv = (k => 1000, kb => 1024, g => 1000000, gb => 1024 * 1024);

# Note that the keys are sorted to put the longer
# matches first in the regex
my $keys = join '|', sort { length $b <=> length $a } keys %conv;

while (<DATA>) {
  print "$_ -> ";
  s/(\d+)($keys)/$1 * $conv{$2}/e;
  print "$_\n";
}
__END__
30k
30kb
10g
10gb
[download]

This gives:

$ ./test.pl
30k
 -> 30000

30kb
 -> 30720

10g
 -> 10000000

10gb
 -> 10485760
[download]

Update: Yes I know that some of the multipliers are wrong. Correcting them is left as as exercise for the reader :)

--
<http://www.dave.org.uk>

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg