I have changed it for the below code but now it is returning only the first element.

$id[0] =  @{$dbh->selectall_arrayref($sql,undef,'no')};
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It's returning all the elements but you are then discarding all but the first. If you want them all, keep them all:

@id =  @{$dbh->selectall_arrayref($sql,undef,'no')};
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It seems I am not able to integrate array, scalar, reference and variables using an undefined "" parameter.

The code explains better than me:

# When it is not typed, the arguments will start as "" blank
# (which means all ids)
@id = "";

# Check typed argument ("-id1", duplicated or none)
foreach my $arg (@ARGV)
{
    if ($arg =~ /^--?id(\d+)$/)
      {
          die "Error: Multiple ids specified: -id@id and $arg.\n" if (
+$id[0]);
 
          @id = $1 ; next;
      }
}

# Exec problem
if ($id[0] eq "")
{   
    my $sql = "SELECT DISTINCT id FROM mytable WHERE active = ?";
    @id =  @{$dbh->selectall_arrayref($sql,undef,'no')};
    print join (", ", @id), "\n";
}
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Output error:

Running SELECT id FROM mytable     WHERE id=? AND salary >= 1 for ARRA
+Y(0x1011e70)
ID: ARRAY(0x1011e70), 1 lines dumped to csvfile1_ARRAY(0x1011e70).csv
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Output error when replaced $id[0] to @id at if conditional: if (@id eq "")

Argument "" isn't numeric in subroutine entry at ./test.pl line 108.
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->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

Please ignore it, I don't have a clue what I am doing

The code was always right, just realized the where clause has bitten me here. lol