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true

is a boolean constant representing the logical state of thuth, the opposite of false.

Even though true looks like a bareword, it is not, because it's one of the exceptional words along with null. It's possible to start an expression with true.

see if check-item returns true any of $list elements 
$seen := false
for $item in $list {
    if (check-item $item) {
        $seen = true
        break
    }
}
true is not a bareword; when passed as a parameter, its type is boolean, not string 
put true
binary expression, $result will be true $result := true or false
can't start bareword with word true, use quotes 
print  true/x  # syntax error
print "true/x" # OK

See Also