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 );
[download]