my($lookahead);

nextline();
while (hasline()) {
        if ($lookahead=~ /^\s*$/) {
                nextline();
                next;
        }
        die "fileformat error: $lookahead" if $lookahead!~ s/^\[([^]]+)\].*/$1/;
        SWITCH: for ($lookahead){
        /^Section1$/ && do { nextline(); section1(); last SWITCH; };
        /^Section2$/ && do { nextline(); section2(); last SWITCH; };
        do { die "unhandled section: $_" };
        }
}

sub section1 {
        while (hasline()) {
                return if $lookahead=~ /^\[/;
                print "S1: ",$lookahead;
                nextline();
        }
}

sub section2 {
        while (hasline()) {
                return if $lookahead=~ /^\[/;
                print "S2: ",$lookahead;
                nextline();
        }
}

sub nextline {
        $lookahead=<DATA>;
}

sub hasline {
        return defined $lookahead;
}
__END__
[Section1]
var1=one
var2=two
var3=three

[Section2]
user1:pass1:50
user2:pass2:51