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SELECT f1 AS x1,f2,f3,f4 as x2, (f4= f5) as x3 FROM %t"

SELECT x1, f2, f3, x2, x3 FROM ( SELECT f1 AS x1,f2,f3,f4 as x2, (f4= 
+f5) as x3 FROM Some_table_2006) UNION (SELECT f1 AS x1,f2,f3,f4 as x2
+, (f4= f5) as x3 FROM Some_tabel_2007)) As Tbl1 ;

SELECT f1 AS x1,f2,f3,f4 as x2, (f4= f5) as x3 FROM

SELECT x1,f2,f3,x2, x3 FROM

SELECT f1,f2,f3,f4 as x2, (f4= f5) as x3 FROM

use strict;
use warnings;
...
$s =~ s/,\s*.*?\s+as/,/gi ;

print "OUT: $s\n";

OUT: SELECT f1, x2, x3 FROM ....