What you're probably looking for is:

do 'mycode.pl';
[download]

As for inserting data into an HTML page, Perl prefers to keep its code separate from text. So instead of dropping code snippets here and there in the page, you'd do something more like this:

use strict;
use warnings;

my ($handle, %hash);

###### Collect template contents and change to string
open($handle, 'template.dat');
$_ = join '', <$handle>;
close($handle);

###### Some dummy values to insert in the template
$hash{'name'} = 'Theodore';
$hash{'age'} = 26;

###### Every instance of %hashkey% is replaced with 
###### the corresponding value
{
no warnings;
s/%(\w+)%/$hash{$1}/g;
}

print;
[download]