Certainly! The only trick is having to enumerate the Custom Fields from the CustomFieldsManager to get the key property you need to call SetField on.

Here's some working code you can start with.

#!/usr/bin/perl

use strict;
use warnings;

use VMware::VIRuntime;

my %opts = (
	vmname => {
	type => "=s",
	variable => "vmname",
	help => "Virtual Machine Name",
	required => 1,
	},
	
	fieldname => {
	type => "=s",
	variable => "fieldname",
	help => "Custom Field Name",
	required => 1,
	},
	
	fieldvalue => {
	type => "=s",
	variable => "fieldvalue",
	help => "CustomFieldValue",
	required => 1,
	},
);

Opts::add_options(%opts);
Opts::parse();
Opts::validate();

Util::connect();

my $vm_name = Opts::get_option("vmname");
my $field_name = Opts::get_option("fieldname");
my $field_value = Opts::get_option("fieldvalue");

my $vm_view = Vim::find_entity_view(view_type => "VirtualMachine",
									filter => { 'name' => $vm_name});
									
unless ($vm_view) {
		Util::disconnect();
		die "Failed to find VM by name: $vm_name\n";
}

my $CustomFieldsManager = Vim::get_view( mo_ref => Vim::get_service_content()->customFieldsManager );

my $field_key = FindCustomFieldKey($CustomFieldsManager, $field_name);

unless ($field_key) {
	my $new_custom_field = CreateVMFieldDefinition($CustomFieldsManager, $field_name);
	$field_key = $new_custom_field->key;
} 

SetVMFieldValue($CustomFieldsManager, $vm_view, $field_key, $field_value);
	
sub FindCustomFieldKey {
	
	my ($cfm, $field_name) = @_;
	my ($field_key, @custom_fields);
	if ($cfm->field) {
		@custom_fields = @{$cfm->field};
	}
	else {
		# No custom fields defined, return undef
		return undef;
	}
	
	foreach my $field ( @custom_fields ) {
		if ($field->name eq $field_name) {
			$field_key = $field->key;
			last;
		}
	}
	
	# will be undef if no matching field to $field_name is found
	return $field_key;
}
	
sub CreateVMFieldDefinition {
	
	my ($cfm, $field_name) = @_;
	
	print "Adding Custom Field \'$field_name\'...\n";
	my $new_cf = $cfm->AddCustomFieldDef(
		name => $field_name,
		moType => "VirtualMachine",
		fieldDefPolicy => undef,
		fieldPolicy => undef);
		
	return $new_cf;
}

sub SetVMFieldValue {
	
	my ($cfm, $entity, $field_key, $field_value) = @_;
	
	print "Setting Custom Field for Virtual Machine...\n";
	$cfm->SetField(
		entity => $entity,
		key => $field_key,
		value => $field_value );
		
	return;
}

AWESOME ! ! ! :smileygrin:

Thank you so much stumpr!!