my @index;
while (<BIGLOG>){
    my $offset = tell BIGLOG; ### This offset is the start of the *next* line!
    my $epoch = ( /^\s*#/ or /^\s\n/ or $_ !~ /^\s*\d/ ) ? 0 
        : Mktime( unpack 'A4xA2xA2xA2xA2xA2', $_  );
    push @index, pack 'NN', $epoch, $offset;
}

##</code><code>##

my( $offset, @index ) = 0; ## The first lines offset is zero
while (<BIGLOG>){
    my $epoch = ( /^\s*#/ or /^\s\n/ or $_ !~ /^\s*\d/ ) 
        ? 0 
        : Mktime( unpack 'A4xA2xA2xA2xA2xA2', $_  );
    push @index, pack 'NN', $epoch, $offset; ## Pair with previous offset
    $offset = tell BIGLOG;   ## and now get the start of the next line 
}