my $string = <<`EOC`;
   -x exclude=$_ foreach (@excludes);
EOC
[download]

Not really sure I understand, but maybe you want something like this (?)

my @excludes = qw(/usrb /cache /journal);

my $string = join " ", map "-x exclude=$_", @excludes;

print $string;

__END__
-x exclude=/usrb -x exclude=/cache -x exclude=/journal
[download]