use strict;
use warnings 'all';
use Fcntl qw /:DEFAULT :flock :seek/;

my $counter_file = "/tmp/counter";
 
sysopen my $fh => $counter_file, O_RDWR | O_CREAT, 0644
        or die "Failed to open $counter_file: $!\n";
 
flock    $fh => LOCK_EX   or die "Failed to flock $counter_file: $!\n";
chomp (my $counter = <$fh> || 0);
$counter ++;
seek     $fh => 0, SEEK_SET;
print    $fh "$counter\n" or die "Failed to write to $counter_file: $!\n";   
truncate $fh => tell $fh  or die "Failed to truncate $counter_file: $!\n";
close    $fh              or die "Failed to close $counter_file: $!\n";
 
... Do whatever you want with $counter here ....