use Fcntl ':flock';

open my $fh, '+<' , 'visit.txt'
    or die "Error opening ..... : $!";
flock $fh , LOCK_EX;

my $counter = <$fh>;
++$counter;

seek $fh,0,0;
truncate $fh,0;

print $fh $counter

close $fh;

# ... output now