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

##</code><code>##

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 ;

##</code><code>##

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

##</code><code>##

SELECT x1,f2,f3,x2, x3 FROM

##</code><code>##

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

##</code><code>##

use strict;
use warnings;

my $s = "SELECT f1,f2,f3,f4 as x2, (f4= f5) as x3 FROM ...." ;

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

print "OUT: $s\n";

##</code><code>##

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