You could do

my ($query, $sth, $rv);

$query = ...;
$sth   = ...;
$rv    = ...;
...

$query = ...;
$sth   = ...;
$rv    = ...;
...
[download]

I did that in a .t file recently. The complexity of the alternative was immense.

Of course, in your specific case, you could write

my $first_result = $dbh->selectrow_hashref(
   "SELECT something FROM somewhere"
);


my $second_result = $dbh->selectrow_hashref(
   "SELECT somethingelse FROM somewhereelse"
);
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what is actually happening in memory

Two different variables are created, as if they had different names.

$ perl -MO=Concise -e'my $x; my $x; my $y;' 2>&1 | grep padsv
3     <0> padsv[$x:1,4] vM/LVINTRO ->4
5     <0> padsv[$x:2,4] vM/LVINTRO ->6
7     <0> padsv[$y:3,4] vM/LVINTRO ->8
[download]