I've solved it...

It was because Perl was in taint mode. Turning off taint mode remedied the problem. I've now re-written the query like this:

my $point = "POINT( $data{'lng'} $data{'lat'} )";
my $query = $dbh->prepare("SELECT idUSRN FROM USRN WHERE idUSRN > 0 OR
+DER BY ST_Distance_Sphere( ST_GeomFromText( ?, 4326 ), center ) LIMIT
+ 10");
$query->execute($point);
my $usrn = $query->fetchrow_array;
[download]

This works under taint mode...

Previously we discussed a similar issue in Recalcitrant placeholders
haukex managed to diagnose the problem to being specific to Perl 5.16 as explained in Re^11: Recalcitrant placeholders (updated x2)