under TextWrangler it shows as UTF-16 EL

Then it's probably UCS-2le or UTF-16le. (The latter is a superset of the former.)

I am trying to convert it to DOS format using Perl.

Most people would consider "DOS format" to mean encoded using their machine's "ANSI" encoding and using CRLF for line endings.

In the Western world, the "ANSI" encoding is usually Windows-1252 aka cp1252.

perl -pe"BEGIN { binmode STDIN, ':encoding(UTF-16le)'; binmode STDOUT,
+ ':encoding(cp1252)'; }" < file.wide > file.ansi
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Update:

The :crlf layer ends up in the incorrect order. That's not a problem with ASCII-derived encodings, but it is with UTF-16le. You actually need to use a workaround like

# wide_to_ansi.pl file.wide file.ansi

@ARGV == 2
   or die("Incorrect usage\n");

open(my $fh_in, '<:raw:perlio:encoding(UTF-16le):crlf', $ARGV[0])
   or die("Cannot open input file \"$ARGV[0]\": $!\n");

open(my $fh_out, '>:raw:perlio:encoding(cp1252):crlf', $ARGV[1])
   or die("Cannot create output file \"$ARGV[1]\": $!\n");

print($fh_out $_) while <$fh_in>;
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Thank you so much ikegami. That solved my problem and know I know.

how to detect the file format whether it is U-DOS or DOS format via perl code

If you mean UTF-16le by "U-DOS", you can check for the presence of 00 bytes. Each ASCII character will be encoded in the form xx 00.