if (1 == (my @f = S1 || S2)) {
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scalar context is forced upon the return of S1, but not on that of S2:

perl -le '@g = qw(a b c); @h = qw(d e); @f = @g || @h; print @f'
3
perl -le '@g = qw(     ); @h = qw(d e); @f = @g || @h; print @f'
de
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What if S1 returns more than one element?

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}