#!usr/bin/perl -w
use strict;

my %scale= (MB => 1,
            GB => 1000,
            KB => 0.001);
           
sub scale2MB
{
   my $string = shift;
   $string =~ tr/a-z/A-Z/;
   if ($string =~ /^[\d.]+$/){return $string};
   my ($num,$suffix) = ($string =~ /^([\d.]+)\s*(\w+)/);
   return ($num * $scale{$suffix});
}

foreach ("10", "10.5", "10 GB", "1kb", "10.1 GB")
{ 
   print "$_=",scale2MB($_),"\n";
}

my $val = scale2MB("753mb") + scale2MB("50 kb");
print "753MB+50KB is: $val\n";
__END__
prints
10=10
10.5=10.5
10 GB=10000
1kb=0.001
10.1 GB=10100
753MB+50KB is: 753.05