I can reproduce a similar behaviour if I remove

use DateTime;
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from the package where foo is declared. Do you use DateTime in the package?

#!/usr/bin/perl
use warnings;
use strict;

use Data::Dumper;

{   package MyClass;
    use DateTime;

    sub foo {
        my ($self,$input) = @_;
        my @ymd = split("-",$input);

        my $date = {
                    day => $ymd[2],
                    month => $ymd[1],
                    year => $ymd[0],
                   };

        my $dt = eval{DateTime->new($date)};
        return unless $dt;

        $date->{dt} = $dt;
        return $date
    }

    sub new { bless {}, shift }

}

my $dt = 'MyClass'->new;
my $result = $dt->foo('2001-01-01');
print Dumper $result;
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Output:

$VAR1 = {
          'dt' => bless( {
                           'utc_rd_days' => 730486,
                           'rd_nanosecs' => 0,
                           'local_rd_days' => 730486,
                           'utc_rd_secs' => 0,
                           'locale' => bless( {
                                                'native_complete_name'
+ => 'English United States',
                                                'en_territory' => 'Uni
+ted States',
                                                'en_language' => 'Engl
+ish',
                                                'en_complete_name' => 
+'English United States',
                                                'native_territory' => 
+'United States',
                                                'default_time_format_l
+ength' => 'medium',
                                                'default_date_format_l
+ength' => 'medium',
                                                'id' => 'en_US',
                                                'native_language' => '
+English'
                                              }, 'DateTime::Locale::en
+_US' ),
                           'utc_year' => 2002,
                           'formatter' => undef,
                           'offset_modifier' => 0,
                           'local_c' => {
                                          'day_of_quarter' => 1,
                                          'day' => 1,
                                          'day_of_year' => 1,
                                          'hour' => 0,
                                          'second' => 0,
                                          'quarter' => 1,
                                          'minute' => 0,
                                          'year' => 2001,
                                          'month' => 1,
                                          'day_of_week' => 1
                                        },
                           'local_rd_secs' => 0,
                           'tz' => bless( {
                                            'offset' => 0,
                                            'name' => 'floating'
                                          }, 'DateTime::TimeZone::Floa
+ting' )
                         }, 'DateTime' ),
          'month' => '01',
          'day' => '01',
          'year' => '2001'
        };
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