If you need to pull out the list of selected columns then the following may help:

use strict;
use warnings;

my $sel = 'SELECT f1 AS x1,f2,f3,f4 as x2, (f4= f5) as x3 FROM';

$sel =~ /SELECT\s+(.*?)FROM/i or die "Select clause not found in: $sel
+";

my @parts = map {/(\w+)\s*$/; $1} split /\s*,\s*/, $1;

print "SELECT ", join (', ', @parts), " FROM\n";
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Prints:

SELECT x1, f2, f3, x2, x3 FROM
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