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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Yes, actually $attached is a single string whether $code is true or false: in the latter case, it’s the empty string. And that means you get a key/value pair in which the string $attached is the key, and the value is undef — which gives the “Odd number of elements” warning.

The solution is to eval the string:

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

my $body      = 'Hello!';
my $code      =  1;
my $email     = 'addressee';
my $file_name = 'unknown';
my $from      = 'sender';
my $name      = 'A Nonny Mouse';
my $subject   = 'Greeting';
my $attached  =  $code ? "attach_path     => '$file_name', " .
                         "attach_filename => '$file_name', " .
                         "attach_type     => 'application/txt'," : '';

my $success =
   {
       to        => $email,
       from      => $from,
       from_name => $name,
       subject   => $subject,
       body      => $body,
       eval $attached,
   };

dd $success;
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Output:

 2:21 >perl 1655_SoPW.pl
{
  attach_filename => "unknown",
  attach_path => "unknown",
  attach_type => "application/txt",
  body => "Hello!",
  from => "sender",
  from_name => "A Nonny Mouse",
  subject => "Greeting",
  to => "addressee",
}

 2:21 >
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Hope that helps,

Athanasius <°(((>< contra mundum

Iustus alius egestas vitae, eros Piratica,

I didn't notice it was a string, thanks. So it could be done like this, though toolic's answer is cleaner.

my $attached = $code ? { attach_path => $file_name, attach_filename =>
+ $file_name, attach_type => 'application/txt' } : {};
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Thanks for all the help, I like tangent's answer!

use strict;
use warnings;
use Data::Dumper;

my $code = 1;
my $file_name = 'my_file_name';
my $email     = 'to@address.com';
my $from      = 'from@address.com';
my $from_name = 'Me';
my $subject   = 'try_me';
my $body      = 'Here is my message.';

my $attached
    = ($code) ? { attach_path     => $file_name,
                  attach_filename => $file_name,
                  attach_type     => 'application/txt',
                }
    :           {}
    ;

my %args = (
to              => $email,
from            => $from,
from_name       => $from_name,
subject         => $subject,
body            => $body,
%$attached
);

my $success = email(%args);


sub email {
    # Display arguments;
    my %arg_hash = @_;
    print Dumper(\%arg_hash);
    ...
}

OUTPUT:
$VAR1 = {
          'attach_path' => 'my_file_name',
          'from' => 'from@address.com',
          'subject' => 'try_me',
          'from_name' => 'Me',
          'attach_filename' => 'my_file_name',
          'to' => 'to@address.com',
          'attach_type' => 'application/txt',
          'body' => 'Here is my message.'
        };
Unimplemented at C:\Users\Bill\perl\monks\email.pl line 38
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