I am having a look at the source code on this page which I am trying to fill the 'username' field with 'username' http://carcommons.com/posting.php?mode=reply&t=35186 as you can clearly see there is a username field on the page and the html/javascript code has this (my apologies for the length, just trying to show where the javascript is, and how I would need to extract/parse the relevant reference to the username field): 
<script language="JavaScript" type="text/javascript">
<!--
// bbCode control by
// subBlue design
// www.subBlue.com

// Startup variables
var imageTag = false;
var theSelection = false;

// Check for Browser & Platform for PC & IE specific bits
// More details from: http://www.mozilla.org/docs/web-developer/sniffe
+r/browser_type.html
var clientPC = navigator.userAgent.toLowerCase(); // Get client info
var clientVer = parseInt(navigator.appVersion); // Get browser version

var is_ie = ((clientPC.indexOf("msie") != -1) && (clientPC.indexOf("op
+era") == -1));
var is_nav = ((clientPC.indexOf('mozilla')!=-1) && (clientPC.indexOf('
+spoofer')==-1)
                && (clientPC.indexOf('compatible') == -1) && (clientPC
+.indexOf('opera')==-1)
                && (clientPC.indexOf('webtv')==-1) && (clientPC.indexO
+f('hotjava')==-1));
var is_moz = 0;

var is_win = ((clientPC.indexOf("win")!=-1) || (clientPC.indexOf("16bi
+t") != -1));
var is_mac = (clientPC.indexOf("mac")!=-1);

// Helpline messages
b_help = "Bold text: [b]text[/b]  (alt+b)";
i_help = "Italic text: [i]text[/i]  (alt+i)";
u_help = "Underline text: [u]text[/u]  (alt+u)";
q_help = "Quote text: [quote]text[/quote]  (alt+q)";
c_help = "Code display: [code]code[/code]  (alt+c)";
l_help = "List: [list]text[/list] (alt+l)";
o_help = "Ordered list: [list=]text[/list]  (alt+o)";
p_help = "Insert image: [img]http://image_url[/img]  (alt+p)";
w_help = "Insert URL: [url]http://url[/url] or [url=http://url]URL tex
+t[/url]  (alt+w)";
a_help = "Close all open bbCode tags";
s_help = "Font color: [color=red]text[/color]  Tip: you can also use c
+olor=#FF0000";
f_help = "Font size: [size=x-small]small text[/size]";

// Define the bbCode tags
bbcode = new Array();
bbtags = new Array('[b]','[/b]','[i]','[/i]','[u]','[/u]','[quote]','[
+/quote]','[code]','[/code]','[list]','[/list]','[list=]','[/list]','[
+img]','[/img]','[url]','[/url]');
imageTag = false;

// Shows the help messages in the helpline window
function helpline(help) {
    document.post.helpbox.value = eval(help + "_help");
}


// Replacement for arrayname.length property
function getarraysize(thearray) {
    for (i = 0; i < thearray.length; i++) {
        if ((thearray[i] == "undefined") || (thearray[i] == "") || (th
+earray[i] == null))
            return i;
        }
    return thearray.length;
}

// Replacement for arrayname.push(value) not implemented in IE until v
+ersion 5.5
// Appends element to the array
function arraypush(thearray,value) {
    thearray[ getarraysize(thearray) ] = value;
}

// Replacement for arrayname.pop() not implemented in IE until version
+ 5.5
// Removes and returns the last element of an array
function arraypop(thearray) {
    thearraysize = getarraysize(thearray);
    retval = thearray[thearraysize - 1];
    delete thearray[thearraysize - 1];
    return retval;
}


function checkForm() {

    formErrors = false;    

    if (document.post.message.value.length < 2) {
        formErrors = "You must enter a message when posting.";
    }

    if (formErrors) {
        alert(formErrors);
        return false;
    } else {
        bbstyle(-1);
        //formObj.preview.disabled = true;
        //formObj.submit.disabled = true;
        return true;
    }
}

function emoticon(text) {
    var txtarea = document.post.message;
    text = ' ' + text + ' ';
    if (txtarea.createTextRange && txtarea.caretPos) {
        var caretPos = txtarea.caretPos;
        caretPos.text = caretPos.text.charAt(caretPos.text.length - 1)
+ == ' ' ? caretPos.text + text + ' ' : caretPos.text + text;
        txtarea.focus();
    } else {
        txtarea.value  += text;
        txtarea.focus();
    }
}

function bbfontstyle(bbopen, bbclose) {
    var txtarea = document.post.message;

    if ((clientVer >= 4) && is_ie && is_win) {
        theSelection = document.selection.createRange().text;
        if (!theSelection) {
            txtarea.value += bbopen + bbclose;
            txtarea.focus();
            return;
        }
        document.selection.createRange().text = bbopen + theSelection 
++ bbclose;
        txtarea.focus();
        return;
    }
    else if (txtarea.selectionEnd && (txtarea.selectionEnd - txtarea.s
+electionStart > 0))
    {
        mozWrap(txtarea, bbopen, bbclose);
        return;
    }
    else
    {
        txtarea.value += bbopen + bbclose;
        txtarea.focus();
    }
    storeCaret(txtarea);
}


function bbstyle(bbnumber) {
    var txtarea = document.post.message;

    txtarea.focus();
    donotinsert = false;
    theSelection = false;
    bblast = 0;

    if (bbnumber == -1) { // Close all open tags & default button name
+s
        while (bbcode[0]) {
            butnumber = arraypop(bbcode) - 1;
            txtarea.value += bbtags[butnumber + 1];
            buttext = eval('document.post.addbbcode' + butnumber + '.v
+alue');
            eval('document.post.addbbcode' + butnumber + '.value ="' +
+ buttext.substr(0,(buttext.length - 1)) + '"');
        }
        imageTag = false; // All tags are closed including image tags 
+:D
        txtarea.focus();
        return;
    }

    if ((clientVer >= 4) && is_ie && is_win)
    {
        theSelection = document.selection.createRange().text; // Get t
+ext selection
        if (theSelection) {
            // Add tags around selection
            document.selection.createRange().text = bbtags[bbnumber] +
+ theSelection + bbtags[bbnumber+1];
            txtarea.focus();
            theSelection = '';
            return;
        }
    }
    else if (txtarea.selectionEnd && (txtarea.selectionEnd - txtarea.s
+electionStart > 0))
    {
        mozWrap(txtarea, bbtags[bbnumber], bbtags[bbnumber+1]);
        return;
    }
    
    // Find last occurance of an open tag the same as the one just cli
+cked
    for (i = 0; i < bbcode.length; i++) {
        if (bbcode[i] == bbnumber+1) {
            bblast = i;
            donotinsert = true;
        }
    }

    if (donotinsert) {        // Close all open tags up to the one jus
+t clicked & default button names
        while (bbcode[bblast]) {
                butnumber = arraypop(bbcode) - 1;
                txtarea.value += bbtags[butnumber + 1];
                buttext = eval('document.post.addbbcode' + butnumber +
+ '.value');
                eval('document.post.addbbcode' + butnumber + '.value =
+"' + buttext.substr(0,(buttext.length - 1)) + '"');
                imageTag = false;
            }
            txtarea.focus();
            return;
    } else { // Open tags
    
        if (imageTag && (bbnumber != 14)) {        // Close image tag 
+before adding another
            txtarea.value += bbtags[15];
            lastValue = arraypop(bbcode) - 1;    // Remove the close i
+mage tag from the list
            document.post.addbbcode14.value = "Img";    // Return butt
+on back to normal state
            imageTag = false;
        }
        
        // Open tag
        txtarea.value += bbtags[bbnumber];
        if ((bbnumber == 14) && (imageTag == false)) imageTag = 1; // 
+Check to stop additional tags after an unclosed image tag
        arraypush(bbcode,bbnumber+1);
        eval('document.post.addbbcode'+bbnumber+'.value += "*"');
        txtarea.focus();
        return;
    }
    storeCaret(txtarea);
}

// From http://www.massless.org/mozedit/
function mozWrap(txtarea, open, close)
{
    var selLength = txtarea.textLength;
    var selStart = txtarea.selectionStart;
    var selEnd = txtarea.selectionEnd;
    if (selEnd == 1 || selEnd == 2) 
        selEnd = selLength;

    var s1 = (txtarea.value).substring(0,selStart);
    var s2 = (txtarea.value).substring(selStart, selEnd)
    var s3 = (txtarea.value).substring(selEnd, selLength);
    txtarea.value = s1 + open + s2 + close + s3;
    return;
}

// Insert at Claret position. Code from
// http://www.faqts.com/knowledge_base/view.phtml/aid/1052/fid/130
function storeCaret(textEl) {
    if (textEl.createTextRange) textEl.caretPos = document.selection.c
+reateRange().duplicate();
}

//-->
</script>


<form action="posting.php" method="post" name="post" onsubmit="return 
+checkForm(this)">


<tr><td>
<table width="100%" cellspacing="2" cellpadding="2" border="0" align="
+center">
    <tr> 
        <td align="left" style="padding-left:5px;"><span class="gensma
+ll"><b><a href="index.php" class="gensmall">Car Commons Forum Index</
+a>
         
        -> <a href="ferraris-vf89.html" class="gensmall">Ferraris</a><
+/b></span></td>
            </tr>
</table>
<tr><td height="5"><img src="templates/freeMind/images/spacer.gif" bor
+der="0" alt="" height="5" width="1" /></td></tr>  

<tr><td>
<table border="0" cellpadding="3" cellspacing="1" width="100%" class="
+forumline">
    <tr> 
        <th class="thHead" colspan="2" height="25"><b>Post a reply</b>
+</th>
    </tr>
        <tr> 
        <td class="row1"><span class="gen"><b>Username</b></span></td>
        <td class="row2"><span class="genmed"><input type="text" class
+="post" tabindex="1" name="username" size="25" maxlength="25" value="
+" /></span></td>
    </tr>

[download]

so far I have tried if( $form->find_input( 'username' ) ) { $mech->set_visible($username)} else {}, as you can see from the page and html there is a username field called 'username' but all I receive is can't call method find_input on an undefined value

Readmore tags added by GrandFather

In reply to perl and a javascript form field by coder45

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