Close! Instead of using UTC and adding an hour (which could have been done much more simply using $dt->add( hours => 1 )), use +0100 as the time zone.

use DateTime           qw( );
use DateTime::TimeZone qw( );

my $old_tz = DateTime::TimeZone->new( name => "+0100" );
my $new_tz = DateTime::TimeZone->new( name => "Europe/Rome" );

for my $date (
  [ 2020, 12, 2, 1, 0 ],
  [ 2021,  7, 2, 1, 0 ],
) {
   my $dt = DateTime->new(
      year =>   => $date->[0],
      month     => $date->[1],
      day       => $date->[2],
      hour      => $date->[3],
      minute    => $date->[4],
      time_zone => $old_tz,
   );

   say $dt->strftime("%FT%T%z");

   $dt->set_time_zone($new_tz);

   say $dt->strftime("%FT%T%z");
}
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Output:

2020-12-02T01:00:00+0100       ST
2020-12-02T01:00:00+0100       ST used in winter in Europe/Rome
2021-07-02T01:00:00+0100       ST
2021-07-02T02:00:00+0200       DT used in summer in Europe/Rome
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You can use the strings +0100 and Europe/Rome directly; I simply used DT::TZ objects for efficiency.