Another ugly approach based on 1003509 (Update: also at https://stackoverflow.com/questions/2422204/how-can-i-interpolate-the-result-of-a-function-in-a-perl-here-doc):

use strict;
use warnings;

sub MakeLine {
                return "Blah Blah param=$_[0]";
   };

print << "HEREDOC";
First line of heredoc
@{[ MakeLine(3) ]}
@{[ MakeLine(543) ]}
@{[ MakeLine("Some text also") ]}
Last line of heredoc
HEREDOC
[download]

Prints:

First line of heredoc
Blah Blah param=3
Blah Blah param=543
Blah Blah param=Some text also
Last line of heredoc
[download]