while(<MYTEMPLATE>){
  # get the names of the fields on this line  
  my @variables = $_ =~ m/<\?--(\S+?)-->/g;
  for my $variable ( @variables ){
    # replace the tag with the value of the variable
    # if there is one.
    s/<\?--$variable-->/$$variable/g if $$variable;
  }
}