I'd sure like to know how Perl determines ("heuristic guess" says perldoc!) that the file is or is not a binary so that I can do that in Ada.

The method used is described here (perldoc functions -X):

The -T and -B tests work as follows. The first block or so of the file is examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's a -T file. Otherwise, that same portion of the file is examined for odd characters such as strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a -B file; otherwise it's a -T file. Also, any file containing a zero byte in the examined portion is considered a binary file. (If executed within the scope of a use locale which includes LC_CTYPE, odd characters are anything that isn't a printable nor space in the current locale.) If -T or -B is used on a filehandle, the current IO buffer is examined rather than the first block. Both -T and -B return true on an empty file, or a file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file && -T $file.

To see the gory details of the implementation, search for pp_fttext in pp_sys.c in the Perl source code (which is also used for pp_ftbinary).

Command Line Examples

As you can see from the bash command line examples below, the Linux file command is much more complex and sophisticated in that it recognizes a number of specific file formats ... while perl's -B file test just crudely guesses whether a file is binary or text based on a simple heuristic:

$ perl -e '-B q{perl-5.38.0.tar.gz} and warn "binary file\n"'
binary file

$ file perl-5.38.0.tar.gz
perl-5.38.0.tar.gz: gzip compressed data, last modified: Sun Jul  2 22
+:26:17 2023, max compression, from Unix, original size modulo 2^32 97
+505280

$ perl -e '-B q{/usr/bin/perl} and warn "binary file\n"'
binary file

$ file /usr/bin/perl
/usr/bin/perl: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV)
+, dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildI
+D[sha1]=6ecb6a6b0956a41de73c75facebcb500b525b860, for GNU/Linux 3.2.0
+, stripped
[download]

See Also

• perlrun (perldoc)
• feature (perldoc)
• Quote-Like-Operators (perldoc)
• Quoting (Bash Reference Manual)

Updated: Added "Command Line Examples" and "See Also" sections.