use strict;
use warnings;

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 $block_size =~ /^(\d+)([KMG]B)$/;
   $num = $1 * 2 ** $convert{$2};
}
print $num;