Why not try this:

sub update_table{
    my $l_tablename = shift;
    my %l_fields = shift;
    my $l_fieldnames;
    my $l_fieldvalues;
    map {
        $l_fieldnames .= $_;
        $l_fieldvalues .= $l_fields{$_};
    } keys %l_fields;

    my $l_prepare = "INSERT $l_tablename (" . $l_fieldnames . ")\n VAL
+UES(" . $l_fieldvalues . ")";
    return $l_prepare;
}
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BatGnat