use CGI;

my $query = CGI->new();

if ($query->param( "button" ) eq 'first') { display_first(  $query ) }
else                                      { display_second( $query ) }

sub display_first {
  my $query = shift;

  # declare your variables here
}

sub display_second {
  my $query = shift;

  # declare your variables here
}