I'd store multipliers in a hash, which is keyed by the end of the string:

my %MULTIPLIERS = (
    k  => 1024, 
    kb => 1024, 
    g  => 1024 * 1024 * 1024, 
    gb => 1024 * 1024 * 1024, 
);

my $num = '30k'; # or 24g or 29kb, whatever

$num =~ s/\A
        (\d+)
        ([A-Za-z]{0,2})
    \z)/$1/x;
my $key = lc $2;
$mult = exists $MULTIPLIER{$key} ? $MULTIPLIER{$key} : 1;
$num *= $mult;
[download]

Update: added lowercase key so that 'g' and 'G' evaluate to the same hash key.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated