# main part of script ...

# then redo the crc:
rewrite_crc('datafile');

sub rewrite_crc {
    my $file = shift;
    open(SELF,"+<$0")||die $!;
    while(<SELF>){last if /^__END__/}
    print SELF new_crc($file),$/;
    truncate(SELF,tell SELF);
    close SELF;
}

sub new_crc {
    my $file = shift;
    my $crc = 0;
    # calculate new crc here
    $crc = 43256;
    return $crc;
}

__END__
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