You need to use the /m modifier so that ^ can match the start of lines within your string. Otherwise it just matches the start of your string and not lines within it. Also, you might want to use the /x modifier so that you can comment your RE a little bit. Here's a snippet for you to modify as is your wont:

#!/usr/bin/perl -l

$text = do { local $/; <DATA> };
print $1 if $text =~ /
   ^Total\s+Datafile\s+Size\s+MB\s*     # header
   ^-+\s*                               # separator
   ^([\d.]+)                            # the number I want
/mix;

__DATA__

Total Datafile Size MB
----------------------
73602.1406


Total Database Size MB
----------------------
48107.7656


Total Free Space MB
-------------------
25462.8125
[download]

Update: Some other notes. I'm not sure why you're using local *FH; but if you have a modern perl, you can do something like this instead:

open(my $fh, $file_name) or die;
my $text = do { local $/; <$fh> };     # slurp
close $fh;
[download]