the substitution displays the path and not the content of the files

You have to read in the content of the files, e.g.

my $fname_include1 = "../includes/header.html";
my $include1;
{
open my $fh, "<", $fname_include1 or die "Couldn't open include '$fnam
+e_include1': $!";
local $/ = undef;  $include1 = <$fh>; 
}
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