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;
[download]

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 >
[download]

Hope that helps,