->selectall_arrayref() returns a reference to an Array of Arrays

[ [field1,field2,etc],[field1,field2,etc],[field1,field2,etc]] so you need the map to extract the first field from each record. Your query only returns one field but the structure is the same.

my $ref =  $dbh->selectall_arrayref($sql,undef,'no');          
@id = map{ $_->[0] } @$ref;
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poj