Here's an Inline::C implementation with some example usage code

#!/usr/bin/perl

use Inline C;

my $cnt = Counter("counter.txt");

die("there was an error incrementing the counter")
    if $cnt == -1;

print "current count is $cnt\n";

exit(0);

__END__
__C__

int Counter(char* filename) {
    FILE *fh;
    int count;
    char file_count[16];

    if(NULL == (fh = fopen(filename, "r+")) )
        return -1;
    
    if(NULL == fgets(file_count, 16, fh))
        return -1;

    file_count[strlen(file_count) - 1] = '\0';

    count = atoi(file_count);
    count++;

    if(-1 == fseek(fh, 0, SEEK_SET))
        return -1;

    if(fprintf(fh, "%d\n", count) < 0)
        return -1;

    fclose(fh);

    return count;
}
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I haven't implemented flock() as I wasn't sure how portable it would be :-/
HTH

_________
broquaint