Use an editor. Open file, type "10", save, close ;-)

Or use Sys::Mmap:

use Sys::Mmap;
{
    new Sys::Mmap my $foo, 2, "bar" or die $!;
    $foo = 10;
}
open my $fh, '<', 'bar' or die $!;
my $foo = <$fh>;
print "read: '$foo'\n";
__END__
read: '10'
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