Backticks just return a string, no magic involved. You can't readline (which is what <$var> does) from a string.

A possible solution is to open an in-memory file handle:

open my $handle, '<', \$query or die "Cannot open in-memory file handl
+e: $!";
while (<$handle>) { ... }
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Or you can use the backticks in list context:

for (`...`) {

}
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Though for this particular problem, the pipe open is still the best (and least memory hungry) solution.