you missed to declare "blurple" as of type RGBColor

use strict;
use warnings;


warn "running";



package RGBColor {

    use fields qw(red green blue);

    sub new {
        my RGBColor $self = shift;
        my %params = @_;

        unless (ref $self) {
            $self = fields::new($self);
        }

        %{$self} = %params;
        return $self;
    }
}

my RGBColor $blurple = RGBColor->new(red => 75, blue => 200, green => 
+0);

print "blurple's blue is $blurple->{blue}\n";

$blurple->new(red => 175, blue => 100, green => 100);


print $blurple->{orange};               ### BANGS AT COMPILETIME

print "blurple's blue is $blurple->{blue}\n";
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OUTPUT

No such class field "orange" in variable $blurple of type RGBColor at 
+d:/tmp/pm/t_fields_pm.pl line 35.
Compilation exited abnormally with code 255 at Sun Feb 14 23:04:10
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runs fine after uncommenting the orange line

running at d:/tmp/pm/t_fields_pm.pl line 5.
blurple's blue is 200
blurple's blue is 100
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