If the eval succeeds, that is, no compilation errors, and no fatal errors or dies, $@ will be *empty*. Furthermore, what you print in an eval, you can't capture easily. By default, it'll go to stdout. What you probably want is something like:

#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    s/<# \{(.*?)\} #>/$1/eeg;
    print;
}

__DATA__
<# { $a = 3; $b = 3; if ($a == $b) { "a=b"; } } #>
<# { 4*3;  } #>
[download]

Which prints:

a=b
12
[download]

Note the differences:

• In the pattern, the parens are inside the braces.
• The replacement part is just $1.
• There are two /e modifiers.
• The data doesn't have prints.

Abigail