I expect you need to make values be a list, and do something like this:

  my @values = ( 1,2,3,4,5 );
  my $placeholders = join( ',', ('?') x scalar @values );
  my $query = "SELECT field1
                 FROM table1
                WHERE field2 IN ( $placeholders )";
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Your prepare and execute calls should then work correctly.

You example is providing a single variable, so I guess DBI is not interpreting "1,2,3,4,5" as a list.