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"SELECT rec,id from memrecs where rec collate latin1_bin rlike '[A-Z]'
+" );

$dbh->do( "UPDATE memrec set rec=LOWER(rec)" );

if ( $_rec =~ /A .. Z/) {
    $_rec =~ tr/A-Z/a-z/g;
    ...
}

if ( $_rec =~ tr/A-Z/a-z/ ) {
    # tr/// returns the number of characters converted
    # so we get here only if there had been at least one A-Z
}