Answering my own question... This seems to do the trick: 
use Path::Tiny;
#... some perl stuff
my $FH = path(".counter")->filehandle({locked=>1, exclusive=>1}, "+>>", ":raw") or die("open .counter: $!\n");
seek($FH,0,0);
my $nr; { local $/; $nr = <$FH> };
$nr ||= "000\n";
die("garbage in .counter\n")  unless $nr =~ /^\d+\n$/;
chomp($nr);
++$nr;
truncate($FH,0);
seek($FH,0,0);
print $FH $nr, "\n";
close($FH) or die("close .counter: $!\n");
# and continuing using $nr
The mode "+>>" creates the file when it does not exist yet, and allows for read and write. The "locked" and "exclusive" mode make sure we're the only one using it.

In reply to Re^3: Atomic update counter in a file by pbijnens
in thread Atomic update counter in a file by pbijnens

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