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.
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