but when $code is true, $attached is a STRING in the OP code. And I couldn't get test code to dereference the string version to include in the new hashref.

Here's some example code which tries and dumps both code=0 and 1:

#!perl
use strict;
use warnings;
use Data::Dumper;

for my $code (0 .. 1) {
    my $attached = $code ? { attach_path => 'etc' } : {};
    my $args = {
        to => 'to email',
        from => 'from email',
        %$attached
    };
    print Dumper($args);
}
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