if (1 == (my @f = S1 || S2)) {

##</code><code>##

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