You've got a static argument list, so there's no way to "delete" them short of editing the code. So instead of hard-coding it what you need to do: build up the argument list in a variable, adding in the onfoo attributes if needed, then pass that dynamic list to the textfield method.

my @textfield_args = ( -name => 'name', ..., -maxlength => 120, );
if( $was_no_change ) {
  push @textfield_args, -onfocus => 'this.value = ''";
  push @textfield_args, -onblur => 'this.value = 'def'";
  push @textfield_args, -onchange => 'delete onfocus, delete onblur";
}

$string .= $cgi->textfield( @textfield_args );
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Thank you Fletch, but I know of a change through the -onchange method of javascript available in CGI so it is not quite that what you coded for me. It would be much easier (IMHO) if there would be a possibility that do the following

-onchange => "override onfocus, override onblur",
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Nevertheless thank you for your help.

Aaah I misunderstood what you were asking, which is a Javascript question and not really a Perl problem (whatever JS library you're using should provide some method of removing or changing event bindings; e.g. jquery would be something like $("textfield[name=name]").unbind( 'focus' ); $("textfield[name=name]").unbind('blur');).

Update: tweaks to sample jquery and link'd to unbind docs

The cake is a lie.
The cake is a lie.
The cake is a lie.

    $string .= $cgi->textfield(
        -name      => 'name',
        -default   => 'def',
        -override  => 0,
        -size      => 20,
        -maxlength => 120, 
        -onfocus   => "if (null == this.changed) this.value = ''",
        -onblur    => "if (null == this.changed) this.value = 'def'",
        -onchange  => "this.changed = true",
    );
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