Just after posting, I realised that I could make it more maintainable:

my %convert = ( KB => 10,
                MB => 20,
                GB => 30 );
my $num;
{
   print "Enter the block size\n (example: 400KB, 10MB, 1GB): ";
   my $block_size = uc <STDIN>;
   print "Invalid block type specified!\n\n" and redo
      unless $block_size =~ /^(\d+)([A-Z]B)$/ # Changed
      and defined $convert{$2};               # here
   $num = $1 * 2 ** $convert{$2};
}
print $num;
[download]

That way, it could be more easily be adapted to include terabytes, by just adding TB => 50 to the %convert hash, and then, a few months later :), the same for whatever comes after terabytes...

dave