The returning clause basically turns your INSERT statement (also) into a SELECT statement, so after ->execute(), you want to ->fetch the result:

...
    my $sqlstatement = "insert into $table ($keylist) values ($qlist) 
+returning id";
    my $sth = $dbh->prepare($sqlstatement);
    $sth->execute(@values) || die "Could not execute statement: $sqlst
+atement $sth->errstr";
    my $id = $sth->fetchall_arrayref->[0]->[0];
...
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