Greetings all,
I do an aweful lot of CGI scripting and have run into this very problem. I would suggest relegating the mail form to a sub within your processing script then you can prepopulate it programatically.
Here is a prepopulation script I wrote early on.
pass it a string containing your form and a reference to a hash containing your data keyed by the values you would find in the corresponding form.
it will return you a string contianing your form prepopulated.
I am positive this can be rewritten more efficiently but I hope this helps.

sub prepopulate_form{
    my ($frm_str, $hashref) = @_;
    my %values = %$hashref;
    my @val_keys = keys(%values);
    for(my $i=0;$i<(scalar(@val_keys));$i++){
        my $name = $val_keys[$i];
        my $frm_elm;
        if( $frm_str =~ m/(<(input|textarea)[^>]*name="$name"[^>]*>)/)
+{
            $frm_elm = $1;
        }elsif( $frm_str =~ m%(<select name="$name".*?/select>)%s ){
            $frm_elm = $1;
        }
        my $new_elm = $frm_elm;
        if($frm_elm =~ m/<input/){
            my $type = $1 if($frm_elm =~ m/type="([^"]*)"/);
            my $value = $1 if($frm_elm =~ m/value="([^"]*)"/);
            if($type eq "radio"){
                $frm_elm = $1 if($frm_str =~ /(<input[^>]*name="$name"
+[^>]*value="$values{$name}"[^>]*>)/);
                $new_elm = $frm_elm;
                $new_elm =~ s/>/ checked="true">/;
            }elsif($type eq "checkbox"){
                if($values{$name} == 1){
                    $new_elm =~ s/>/ checked>/;
                }
            }elsif($type =~ m/text/){
                if($new_elm =~ /value=/){
                    $new_elm =~ s/value="[^"]*"/value="$values{$name}"
+/;
                }else{
                    $new_elm =~ s/>/ value="$values{$name}">/;
                }
            }
        }elsif($frm_elm =~ /<textarea/){
            $new_elm =~ s/(<textarea[^>]*>)/$1 $values{$name}/;
        }elsif($frm_elm =~ /<select/){
            $new_elm =~ s/(value="$values{$name}")/$1 selected/s;
        }
        $frm_elm =~ s/([\-\?\)\(\/])/\\$1/g;
        $frm_str =~ s/$frm_elm/$new_elm/s;
    }
    return $frm_str;
}#end prepopulate_form sub
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