Speaks for itself, isn't perfect tho.

my $printed;
while (<TEMPLATE>) {
  if (/<!-- MAIN -->/ .. /<!-- ENDMAIN -->/) {
    unless ($printed) {
      open FORMTEMPLATE, 'form.txt' or die "cannot find form.txt: $!";
      local $_;
      print while (<FORMTEMPLATE>)
      close FORMTEMPLATE;
      $printed++;
    }
  }
  else {
    print;
  }
}
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