pgsql: replace use of predefined perl vars $a and $b with $x and $y -

Log Message:
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replace use of predefined perl vars $a and $b with $x and $y - per Greg Sabino Mullane

Modified Files:
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pgsql/doc/src/sgml:
plperl.sgml (r2.46 -> r2.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.46&r2=2.47)