On a side note, might I suggest using the CGI module or similar to output HTML, instead of printing it yourself. It will probably make things a lot simpler. Here is an sample of your code rewritten using CGI (plus a few style changes):

use CGI qw(:standard); # use functions instead of OO interface

sub build_select_field_from_db {
    # --------------------------------------------------------
    # Builds a SELECT field from the database.
    my ( $column, $value, $name ) = @_;

    my ( $fieldnum, $found ) = ( 0, 0 );
    for my $i (0 .. $#db_cols) {
        if ($column eq $db_cols[$i]) {
            $fieldnum = $i; $found = 1;
            last;
        }
    }
    if (!$found) {
        return q(error building select field: no fields specified!);
    }
    open my $DB, q(<), $db_file_name or cgierr(qq(unable to open $db_f
+ile_name. Reason: $!));
    my @lines = <$DB>;
    close $DB;
    my @selectfields;
    LINE: foreach my $line (@lines) {
        if ($line =~ /^#/) { next LINE; }       # Skip comment lines.
        if ($line =~ /^\s*$/) { next LINE; }    # Skip Blank Lines.
        chomp $line;

        my @fields = split_decode ($line);
        if(!(List::MoreUtils::any { $_ eq $fields[$fieldnum] } @select
+fields)){
            push @selectfields, $fields[$fieldnum];
        }
    }
    my $output = popup_menu(
        -name    => defined $name ? $name : $column,
        -values  => [ '---', sort @selectfields ],
        -default => $value,
    );
    return $output;
}
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