As with the other two questions you've posted this morning in rapid succession it has pretty much nothing to do with Perl and very minimally anything to do with CGI. You need to actually bother to read the information people have suggested to you about CSS and maybe look at the documentation for the -style parameter to the HTML generation subs in CGI. However it's better practice to separate presentation (the HTML) from code and use something like Template Toolkit instead which makes this even less of a Perl problem.

Just add id or class attribute to your button element and add this rules to your CSS:

/*this is for id*/
#yourID {
    color: green;
    ....;
}
/*this is for class*/
.yourClass {
    color: green;
    ....;
}
[download]

See the CGI documentation on using the -style attribute

I'd suggest removing HTML from Perl by using something like HTML::Template. This means you can edit the HTML/CSS/JavaScript in the template files as normal, then populate it with dynamic data using HTML::Template within your Perl code.

Martin