...

my $step = $cgi->param('step') || 0;

my @functions = (
    \&eins, \&zwei, &\drei
);

{
    no warnings;
    $step = int $step;
}
$step = 0 if $step > 2;
my $function = $functions[$step] or die "invalid $step";

my $hidden = $cgi->hidden( -name => 'step', -value => $step+1 );
$function->();

sub eins {
    # code von eins.cgi
    ...
    print $hidden;
}

sub zwei {
    # code von zwei.cgi
    ...
    print $hidden;
}

...