Do:

for (keys %{$self->{SQL}}) {
    my $closure = $_;
    $self->{$_} = $self->{DBO}->prepare($self->{SQL}->{$_});
    $self->{"f$_"} = sub {
                            my $self = shift;
                            my @res;
                            $self->{ $closure }->execute(@_);
                            while (my $dbrow = $self->{ $closure }->fe
+tchrow_hashref()) {
                                push @res, \$dbrow;
                                }   
                            return \@res;
                            };  
}
[download]

What you have at the moment is like this:

undef %h; 
for( qw[ the quick brown fox ] ){ 
    $h{ $_ } = sub { print "$_"; }; 
}; 

pp \%h;  
do {
  my $a = { brown => sub { "???" }, fox => 'fix', quick => 'fix', the 
+=> 'fix' };
  $a->{fox} = $a->{brown};
  $a->{quick} = $a->{brown};
  $a->{the} = $a->{brown};
  $a;
}

$_='joe';

for my $key ( keys %h ) { 
    $h{ $key }->(); 
};;

joe
joe
joe
joe
[download]

Ie. The subroutine is referencing the variable $_; and when it is called, it will use whatever the current value of $_ happens to be at the time.

What you need is this which creates a separate closure for each subroutine instantiated in the loop:

undef %h; 
for( qw[ the quick brown fox ] ){ 
    my $closure = $_; 
    $h{ $_ } = sub { print $closure; }; 
}; 

pp \%h; 
{
  brown => sub { "???" },
  fox   => sub { "???" },
  quick => sub { "???" },
  the   => sub { "???" },
}

$_='joe'; 

for my $key ( keys %h ) { 
    $h{ $key }->(); 
};;
the
fox
brown
quick
[download]