Use a lookup table:

%lookup = map{ 2**$_ => $_ } 0 .. 63;;

$flags = 0;;

$flags |= 1<<(rand 64);;

print "The flag set is:", $lookup{ $flags };;
The flag set is: 12
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Some form of loop is necessary to avoid an impossibly large lookup table:

$flags = int rand( 2**64 );;

print for grep $flags & 1 << $_, 0 .. 63;;
49
50
51
54
55
57
58
59

printf "%u\n", $flags;;
1066790161733386240

$t = 0; print $t += 1<<$_ for  grep $flags & 1 << $_, 0 .. 63;;
562949953421312
1688849860263936
3940649673949184
21955048183431168
57983845202395136
202099033278251008
490329409429962752
1066790161733386240
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