I pretty much agree with ikegami, but I can imagine what you're looking for.

Use a hidden input field to let your script know to process the form. Then you can check for the CGI parameter with your script. If it's there, execute your subroutine...

use strict;
use warnings;

#...

my $cgi = new CGI;

if ( $cgi->param('runsub') ) {
    run_subroutine( $cgi );
}
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-- GhodMode