Open the file RW ('+<'. See perlopentut for more info.)

Use sysseek to move to the byte ("bit") of interest.

Use syswrite (syswrite FH, $bitvalue, 1;) to write the "bit" in question.

#! perl -slw
use strict;
use Fcntl qw[ SEEK_SET SEEK_CUR SEEK_END ];

open OUT, '> :raw', 'test.dat' or die $!;
print OUT 'descriptor';
print OUT '1' x 1000;
close OUT;

my $descOffset = length( 'descriptor' ) + 2; ## May vary with OS defin
+ition of "\n";

open RW, '+< :raw', 'test.dat' or die $!;

for( my $bitPos = 0,; $bitPos < 1000; $bitPos += 2 ) { 
    ## Read bit[ $bitPos ]
    sysseek RW, $descOffset + $bitPos, SEEK_SET;
    my $bit;
    sysread RW, $bit, 1; 
    print "bit[ $bitPos ] = $bit";

    ## Update bit[ $bitPos ]
    sysseek RW, $descOffset + $bitPos -1, SEEK_SET; 
      ## -1 gives zero-based bit positions.
    syswrite RW, '0', 1; 
}

close RW;
[download]

Yes, I plan to repleace the same length of data as i intend to write i.e. just changing the 0 or 1 to a 1 or 0.
Thanks for the help.
Vince