There is still a difference though, note that the DateTime object you've built is in the special "floating" time zone, and setting a time zone on it won't adjust the time value:

use warnings;
use strict;
use DateTime::Format::Strptime;

my $strp = DateTime::Format::Strptime->new(
    on_error=>'croak', pattern => '%Y-%m-%d %H:%M:%S%z');
my @dt = (
    $strp->parse_datetime('2017-06-10 21:55:00+02'),
    DateTime->new(year=>2017,month=>6,day=>10,
        hour=>21,minute=>55,second=>0),
    $strp->parse_datetime('2017-06-10 21:55:00+00'),
);

for my $i (0..$#dt) {
    print "$i: ",$dt[$i]->strftime('%Y-%m-%d %H:%M:%S %Z (%z)'),"\n";
    $dt[$i]->set_time_zone('Europe/Paris');
    print "$i: ",$dt[$i]->strftime('%Y-%m-%d %H:%M:%S %Z (%z)'),"\n";
}

__END__

0: 2017-06-10 21:55:00 +0200 (+0200)
0: 2017-06-10 21:55:00 CEST (+0200)
1: 2017-06-10 21:55:00 floating (+0000)
1: 2017-06-10 21:55:00 CEST (+0200)
2: 2017-06-10 21:55:00 UTC (+0000)
2: 2017-06-10 23:55:00 CEST (+0200)
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