IO::String is useful for this kind of task.

use IO::String;

...

$data =~ s/<# (\{.*?\}) #>/evalCap($1)/eg;

sub evalCap
{
    my $code = shift;
    my $str;
    my $str_fh = IO::String->new($str);
    my $old_fh = select $str_fh;

    eval $code;    

    select $old_fh;
    $str;
}
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Update:
The regular expression can be simplified: s/<# {(.*?)} #>/....
As Abigail implies, the braces aren't needed in the eval. Nor in this case do they need to be escaped.

- nashdj