use strict;

# get the list of files to work on:

my $tkdir = ".";   # put a real path here
chdir $tkdir or die "chdir $tkdir: $!\n";  # this makes things easier below

opendir( TKS, "." );
my @files = grep { -f } readdir TKS ; # only keep the things you want here
closedir TKS;

# read 10 bytes at fixed offset in each file, set replacement values:

my %edit_list;
my $counter = 0;
my $fixed_offset = 50;  # put your real byte offset value here

for my $file ( @files ) {
    open( FH, '<', $file ) or do {
        warn " skipped $file: $!\n";
        next;
    };
    binmode FH;
    seek FH, $fixed_offset, 0;
    read FH, $_, 10;
    close FH;
    my $replace = sprintf( "foobar%04d", ++$counter );
    $edit_list{$file} = [ $_, $replace ];
}

# now, go through the files and edit each one

for my $file ( keys %edit_list ) {
    local $/;  # sets input record separator to undef for slurp mode
    open( FH, '<', $file );
    $_ = <FH>;
    close FH;

    s/\Q$edit_list{$file}[0]/$edit_list{$file}[1]/g;

    open( FN, '>', "$file.edited.$$" )   # write to a different name, just to be safe
        or die $!;
    print FN;
    close FN;
}