Because they are considered statements and not functions, while and foreach are documented in perldoc perlsyn.

while doesn't do any binding except when calling readline or glob (either by name or by using the <...> syntax):

while (<STDIN>) { ... }
while (readline(STDIN)) { ...}

are equivalent to: while (1) { last unless defined($_ = <STDIN>); ... 
+}

while (glob("*.c")) { ... }
while (<*.c>) { ... }

are equivalent to: my @list = glob("*.c"); for (@list) { ... };
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