my $mode = param('mode');

# Main
if (not defined $mode) {
 # ...
}

# After upload
elsif ($mode eq 'secondstage') {
 # ...
}

# Final stage
elsif ($mode eq 'finalstage') {
 # ...
}

##</code><code>##


sub main {
 # Main page
}

sub secondstage {
 # After upload
}

sub finalstage {
 # final stage
}

sub _invalid {
 # Output error message: no such mode
}

my $modes = {
 ''          => \&main,
 secondstage => \&secondstage,
 finalstage  => \&finalstage,
 _invalid    => \&invalid
};

my $mode = param('mode');
$modes->{ exists $modes->{$mode} ? $mode : '_invalid' }->();