Sounds like you want

for_pairs { $att{$a} = $b } /\W*(\w+)="([^"]+)"/ig;
[download]

Let us know if you need help writing for_pairs.

Ah - Thanks for pointing out that this could be done that way.

This works (Plagerized code):

use strict;
use Data::Dumper;

sub for_pairs (&@) {
    unless (@_&1) {
        warn "Odd";
        push @_, undef;
        goto \&for_pairs;
    }
    my $code = shift;

    # Localise $a and $b
    # (borrowed from List-MoreUtils)
    my ($caller_a, $caller_b) = do {
        my $pkg = caller();
        no strict 'refs';
        \*{$pkg.'::a'}, \*{$pkg.'::b'};
    };
    local(*$caller_a, *$caller_b);

    no warnings;
    
    if (wantarray) {
        # list context
        map {(*$caller_a, *$caller_b) = \splice(@_, 0, 2); $code->()} 
+(1..@_/2)
    }
    elsif (defined wantarray) {
        # scalar context
        # count number of returned elements
        my $i=0;
        # force list context with =()= for the count
        $i +=()= $code->() while (*$caller_a, *$caller_b) = \splice(@_
+, 0, 2);
        $i
    }
    else {
        # void context
        $code->() while (*$caller_a, *$caller_b) = \splice(@_, 0, 2);
    }
}
#---------------------------------------------
my $Incoming = 'uno="one" Blah blah dos="two"  * More Noise* tres="thr
+ee"';
my %att= (PREEXISTING=>"val1", ANOTHER=>"Val2");

for_pairs { $att{$a} = $b } $Incoming=~m/\b(\w+)="(.+?)"/g;
print Dumper \%att;
[download]

I don't think I will use it in my current app - I'll stick with the one-liner above.
For future apps with access to Modules, It would be better to go with an established module that provides the function.
Anyway - This has been a fun exercise. Thank you.

     Syntactic sugar causes cancer of the semicolon.        --Alan Perlis