my @pos = (2,4,6,9) ;
my $bits = '' ;
foreach (@pos) { vec($bits, $_, 1) = 1 ; } ;
($bits = unpack('b*', $bits)) =~ s/0+$// ;
my @bits = split(//, $bits) ;
print "(", join(',', @bits), ")\n" ;
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...when dealing with bit vectors vec is your friend, as are unpack/pack b/B and (occasionally) reverse.