Just a small comment on reading in a whole file at once. It's not efficient to read in linewise just to glue them together afterwards. Instead do it like this:

my $template;
{
  local $/; # undef $/ => file slurp mode
  open (FILE, '<', 'file.txt') or die "Couldn't open file.txt: $!";
  $template = <FILE>;
  close FILE;
}
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-- Hofmator