First of all, I wouldn't put headers like that in a here document, or another type of string. There are about fifty billion mail related modules on CPAN. There's got to be at least one doing what you want. I'd look at MIME::Lite.

Second, why can't you put an if statement there?

print <<"EOT";
...
@{[$foo ?"Blah blah bla" : "Other blah; more other blah"]}
...
EOT
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Third, you don't need an if inside body, you can put it outside:

my $headers = $foo ? "Blah blah bla"
                   : "Other blah; more other blah";
print <<"EOT";
...
$headers
...
EOT
[download]