From type/Mu

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method so()

Evaluates the item in Boolean context (and thus, for instance, collapses Junctions), and returns the result. It is the opposite of not, and equivalent to the ? operator.

One can use this method similarly to the English sentence: "If that is so, then do this thing". For instance,

my @args = <-a -e -b -v>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if $verbose-selected.so {
    say "Verbose option detected in arguments";
} # OUTPUT: «Verbose option detected in arguments␤»

The $verbose-selected variable in this case contains a junctions, whose value is any(any(False, False), any(False, False), any(False, False), any(True, False)). That is actually a truish value; thus, negating it will yield False. The negation of that result will be True. so is performing all those operations under the hood.

From type/Bool

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multi sub prefix:<so>(Mu --> Bool:D)

Coerces its argument to Bool, has looser precedence than prefix:<?> .

From language/operators

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multi sub prefix:<so>(Mu $x --> Bool:D)

Evaluates its argument in Boolean context (and thus collapses Junctions), and returns the result.