The first program is buggy. You decode without ever encoding. You'd get a "wide character" warning for some inputs if you had warnings on.

The second program is also buggy, but for a different reason. You presume a line ends at byte 0x10, but that's not true.

And they output differently. The first outputs a mix of iso-latin-1 and UTF-8. The second program outputs UTF-16le or UTF-16be, probably the latter.

use strict;
use warnings;

use open ':std', ':locale';

my $file = shift;
my $enc = "UTF-16";
open(my $fh, "<:encoding($enc)", $file) || die("Can't: $!");
while ( <$fh> ) {
    print;
}
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use strict;
use warnings;

use open ':std', ':locale';

my $file = shift;
my $enc = "UTF-16";
my $file = do {
   open(my $fh, "<:raw", $file) || die("Can't: $!");
   local $/; 
   <$fh>
};
my $str = decode('UTF-16', $_);
print $str;
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The output seems to have a space between every char

That's usually a sign of UTF-16le/UTF-16be/UCS-2le/UCS-2be being treated as ASCII or a derivative like iso-latin-1 or UTF-8.