Anon, with your redirection you will mix stderr messages from PARSING your input script (as perl, with perl_parse()) with stderr messages from RUNNING it (if it is indeed perl, with perl_run()). What I mean is that if your script, for example, fails to find a file during runtime, this will go to the logger file, mixed with all the possible syntax errors while assessing if perl or python. Well it is a solution.

However, if you use the latest version of what I provided (see the modified C program in here: Re: did you mean python), you will have the syntax errors (via stderr) redirected to a logger file and then stderr will be reinstated to what it usually is so that you will have the runtime errors to your terminal. This behaviour is controlled by an env variable. You can modify the C program to provide the logger filename with another env variable.

bw, bliako

/*
   python.c  - 27/01/2026
   by  bliako (bliako at cpan org)
   for https://perlmonks.org/?node_id=11167241

   Run it either with a Perl CLI and a Perl script,
   like you would normally do with perl script.pl,
   perl -e ... etc. or with a python CLI and a python script.
   If the script does not parse as valid Perl code,
   it will pass the CLI and the script to a python interpreter.
   CAVEAT: it will fallback to python if your script is Perl
           but has syntax errors.

   compile it with:
     gcc python.c -o python `perl -MExtUtils::Embed -e ccopts -e ldopt
+s`
*/

#include <EXTERN.h>               /* from the Perl distribution     */
#include <perl.h>                 /* from the Perl distribution     */

static PerlInterpreter *my_perl;  /***    The Perl interpreter    ***/

/* You gotta love Perl */
int main(int argc, char **argv, char **env)
{
    PERL_SYS_INIT3(&argc,&argv,&env);
    my_perl = perl_alloc();
    perl_construct(my_perl);
    PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
    int ret = perl_parse(my_perl, NULL, argc, argv, (char **)NULL);
    if( ret ){
            perl_destruct(my_perl);
            perl_free(my_perl);
        // add here language detection logic,
        // various options available open source
        fprintf(stderr, "I guess it is &*^#$* pyton /sic/ ...\n");
        execvpe("/usr/bin/python", argv, env);
    }
    printf("It parses as Perl so I am running it ...\n");
    perl_run(my_perl);
        perl_destruct(my_perl);
        perl_free(my_perl);
    PERL_SYS_TERM();
    exit(EXIT_SUCCESS);
}
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