use strict;
use warnings;

my $fileStr = <<FILE;
=wibble
The wibble section
=plonk
the plonk section
=foo
the foo section
FILE

open my $fin, '<', \$fileStr;
my $obj = bless {fin => $fin};

$obj->parseStructure($fin);
$obj->parseBlock('foo');
$obj->parseBlock('wibble');
$obj->parseBlock('bar');


sub parseStructure {
    my ($self) = @_;
    my $currBlock;

    while (1) {
        my $start = tell $fin;
        my $line  = <$fin>;

        next if defined $line && $line !~ /^=(\w+)/;

        $self->{blocks}{$currBlock}{length} =
            $start - $self->{blocks}{$currBlock}{start}
            if defined $currBlock;

        $currBlock  = $1;
        $self->{blocks}{$currBlock}{start} = $start if defined $line;
        last if eof $fin;
    }
}


sub parseBlock {
    my ($self, $blockType) = @_;

    if (!exists $self->{blocks}{$blockType}) {
        print "No '$blockType' blocks in file\n";
        return;
    }

    seek $self->{fin}, $self->{blocks}{$blockType}{start}, 0;
    read $self->{fin}, my $block, $self->{blocks}{$blockType}{length};
    print "$blockType:\n$block\n";
}

##</code><code>##

foo:
=foo
the foo section

wibble:
=wibble
The wibble section

No 'bar' blocks in file