Only create one $c. Then pass it to validate. CGI can get tempermental when you keep creating new ones.

#!/usr/bin/perl

use strict;
use HTML::Template;
use MIME::Lite;
use CGI;
use CGI::Carp qw/fatalsToBrowser/;
use Data::Dumper;

my $c = new CGI;

validate($c,  qw/ email phone/ );

print $c->header,$c->start_html('everythings ok');


sub validate {
  my $c = shift;
  my @params = @_;
  my @warning;
  
  for ( @params ) {
    push (@warning, { 'missing' => $_  }) unless $c->param($_);
  }
  if ( @warning ) {
    print $c->header;
    my $template = HTML::Template->new(filename => './templates/warnin
+g.html');
       $template->param( warnings => \@warning );
       print $template->output();
       print Dumper @warning;
    exit;
  }
}
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