This avoids the temporary, but I'm not sure that it is any better for doing so:

use strict;
use Data::Dumper;

my $Incoming = 'uno="one" Blah blah dos="two"  * More Noise* tres="thr
+ee"';

my %att= (PREEXISTING=>"val1", ANOTHER=>"Val2");

$att{ uc $_->[ 0 ] } = $_->[ 1 ] for map[
    m[(\w+)="([^"]+)"] 
], $Incoming =~ m[\W*(\w+="[^"]+")]ig;

print Dumper \%att;
[download]