There are several ways to do this, depending on how much validation you want to add. A lightweight method without any validation is to add a BUILDARGS routine to your class (see Moose::Manual::Construction): 
sub BUILDARGS {
    my $class = shift;
    my %args = ref $_[0] ?  %{$_[0]} : @_;
    $args{code} = uc $args{code} if exists $args{code};
    return \%args;
}

[download]
A more idiomatic method is to add a Moose type for the currency code, check for its validity, and then allow lowercase input by converting it under the hood by Moose coercion. Here's a complete example: 
package UpperCaseDemo;
use Moose;
use Moose::Util::TypeConstraints;
with 'MooseX::Getopt';

subtype 'CurrencyCode'
    => as 'Str'
    => where { /^[A-Z]{3}$/ }
    => message { 'Currency codes should be three characters' },
    ;

coerce 'CurrencyCode'
    => from 'Str'
    => via { uc }
    ;

has 'code' => (
    is => 'ro', isa => 'CurrencyCode',
    coerce => 1,
);

1;

# -----------
# Usage: lookup --code abc or lookup --code ABC
#
package main;
my $cc = UpperCaseDemo->new_with_options();

print "code is ", $cc->code // 'not set', "\n";

[download]
BTW: I don't think that triggers are supposed to be allowed to change readonly attributes. Where did you read that?

In reply to Re: pre-preprocess Moose args in constructor by haj
in thread pre-preprocess Moose args in constructor by Anonymous Monk

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