my %hash;
sub list { print join( ' ', sort keys %hash ), $/ }
for( $hash{for} ) {
    # Don't do a single thing to $_
}
list(); # 'for' entry autovivified unconditionally
sub maybeset { for( $_[0] ) { $_ = $_[1] if 1 < @_ }; list() }
maybeset( $hash{maybe} );        # no 'maybe', despite the for() in the sub
maybeset( $hash{set}, 'hello' ); # now 'for set'