use strict;
use warnings;
use Data::Dumper;

my $structure = main(join '',<DATA>);
print Dumper($structure);

sub main {
    my ($s, $c, %hash) = $_[0];
    while ($s =~ /START PAGE(?: (\w+))?\s+(.*?)\s+END PAGE/gs) {  
        $hash{$1 ? $1 : ++$c} = page($2);
    }
    return \%hash;
}

sub page {
    my ($s, $c, %hash) = $_[0];
    while ($s =~ /START QUESTION(?: (\w+))?\s+(.*?)\s+END QUESTION/gs) {
        $hash{$1 ? $1 : ++$c} = question($2);
    }
    return \%hash;
}

sub question {
    my ($s, %hash) = $_[0];
    ($hash{'label'}) = $s =~ /LABEL (.*)/;
    $s =~ /START CHOICES\s+(.*?)\s+END CHOICES/s;
    for (split / *\n */, $1) {
        push @{$hash{'choices'}}, [split / /, $_, 2];
    }
    return \%hash;
}

__DATA__
START PAGE p1
    START QUESTION 4B
        LABEL Do you like your pie with ice cream?
        START CHOICES
            1 Yes
            2 No
        END CHOICES
    END QUESTION
    START QUESTION 4C
        LABEL Do you like your pie with whipped cream?
        START CHOICES
            1 Yes
            2 No
        END CHOICES
    END QUESTION
END PAGE