Firstly you don't need all those my $c =..., but the error is because you sent a hash not a hash reference (also print $template->output).:

sub validate {
  my @params = @_;
  my @warning;

  for ( @params ) {
    my %h = ( missing => $_);
    push (@warning, \%h ) unless $c->param($_); 
  }
  if ( @warning ) {
    print $c->header;
    my $template = HTML::Template->new(filename => './templates/warnin
+g.html');
    #don't use die $! while testing HTML::Template has it's own error 
+handling
    $template->param( warnings => \@warning );
    print $template->output();
    exit;
  }
}
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