This was quite a bit shorter before I threw in all the comments.

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %vars;

my $data = do{ local $/; <DATA> };

# first, get the items that have a set of UI elements
while ( $data =~ s/
        (\S+)      # non-spaces -- the type
        \s+        # spaces
        (\S+)      # non-spaces -- the name
        \s+        # spaces (including newlines)
        \<         # opening angle thingy
        \s+        # spaces (including newlines)
        (          # start capturing the UI stuff
         (?:       # grouping UI lines
          \S+      # non-spaces -- the UI stuff type
          \s+      # spaces
          UI\S+    # UI name
          \s*=\s*  # equal sign, maybe spaces
          .*       # anything except a newline
          \;\s*    # traling semicolon, optional spaces (including new
+line)
         )+        # UI lines repeat
        )          # capture all the UI lines
        \s*        # optional spaces (including newline)
        \>         # ending angle thingy
        \s*=\s*    # equal sign, maybe spaces
        (.*)       # the main variable value
        \;         # trailing semicolon
        //mx ) {
    my ( $main_type, $main_varname, $ui_stuff, $main_value )
     = ( $1,         $2,            $3,        $4          );

    my $ui_ref = {};

    # pull out the individual UI elements
    while ( $ui_stuff =~ s/
            \S+       # non-spaces -- the ui element type
            \s+       # spaces
            UI(\S+)   # UI element name
            \s*=\s*   # equal sign, maybe spaces
            (.*)      # UI element value
            \;        # trailing semicolon
            //mx ) {
        my ( $name, $value ) = ( $1, $2 );
        # lowercase the name
        $name =~ tr/A-Z/a-z/;
        $ui_ref->{$name} = $value;
    }

    $vars{ $main_varname }{ type }  = $main_type;
    $vars{ $main_varname }{ value } = $main_value;
    $vars{ $main_varname }{ ui }    = $ui_ref;
}    

# get the rest of the stuff that doesn't have UI thingies
while ( $data =~ s/
        (\S+)     # type
        \s+       # spaces
        (\S+)     # name
        \s*=\s*   # equal, maybe spaces
        (.*)      # value
        \;        # trailing semicolon
        //mx ) {
    my ( $type, $name, $value ) = ( $1, $2, $3 );
    $vars{$name}{value} = $value;
    $vars{$name}{type} = $type;
}

print Dumper( \%vars );

__DATA__
float myVarA
<
float UIMin = 1;
float UIMax = 0;
float UIStep = .001;
string UIWidget = "slider";
> =  0.5f;

float myVarB = 1.0;

float4 myVarC = {1,0,0,1};
[download]

Output:

$VAR1 = {
          'myVarA' => {
                        'ui' => {
                                  'step' => '.001',
                                  'min' => '1',
                                  'max' => '0',
                                  'widget' => '"slider"'
                                },
                        'value' => '0.5f',
                        'type' => 'float'
                      },
          'myVarC' => {
                        'value' => '{1,0,0,1}',
                        'type' => 'float4'
                      },
          'myVarB' => {
                        'value' => '1.0',
                        'type' => 'float'
                      }
        };
[download]

There are a few loose ends still (the trailing "f" on the float value, for instance), but those should be easy to clean up.