But from that thread seems to refer to a (possible) bug in tied arrays, whereas I found a (possibly) anomalous behaviour with tied scalars.

However, this particular post from that thread brings me to

• warning: tied vars may evaluate twice in logical expressions,
• warning: bug with tie (5.6.1).

Both hint at the fact that tied vars may... well, evaluate twice under some circumstances. In particular the former seems to refer to a different situation, but the latter portraits exactly the same one as mine. But the same article claims that it was a 5.6.1 specific bug and that it should have been patched - here's the relevant quote:

I searched p5p and it appears to have been reported and patched in development versions --- I just wanted to let people here know so they might avoid some confusion if they run into it. A relevant p5p link (from which the patch is linked) is: here

May it be that the patch never made its way in the next releases or that the same bug has been reintroduced subsequently?

Remark: more importantly, it seems that the FETCH method is not called twice:

#!/usr/bin/perl -l

use strict;
use warnings;

sub TIESCALAR { bless \my $i, shift }
sub FETCH { warn "FETCH called!\n"; ${shift,}++ }

tie my $s, 'main';
print "$s$s$s";

__END__
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FETCH called!
FETCH called!
FETCH called!
112
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