my %section;
my $hdr_string;  # (update: this was intended to be a scalar)

while (<>) {
    next unless ( /\S/ ); # skip blank lines
    if ( /^\[(.*)\]/ ) {  # this is a header line
        $hdr_string = $1;
    }
    else {                # this is a data record
        push @($section{$hdr_string}}, $_;
    }
}

# All the lines under a given "header" are now in
# the array @{$section{header}}; at this point you can
# loop over the sections to manipulate the data records 
# as appropriate -- e.g.:

for my $header ( keys %section ) {
    for my $rec ( @{$section{$header}} ) {
        my @recfields = split( /\s+/, $rec );
        # ... do whatever is to be done with field data ...
    }
}