I have a bunch of mathematical projects, including lots of perl code. For these I usually use a custom-built perl optimized for speed and with a bunch of useful modules installed - locally that lives as a symlink under '/opt/maths/perl'.

That then leaves me with a tension: for my own convenience I want the shebang line on the perl programs to point to this custom perl, but that then makes like difficult for others when I make the code available on github, particularly since they are designed to be run in-repository without an installation step.

Today I finally decided to do something about that. I want the designated scripts (and only those) to use the custom perl when I run them, but to be usable by others using standard setups, and to be fairly robust against user error. And I want any preamble to be as terse as possible. After a bunch of back and forth with both Claude and Gemini, I ignored all their suggestions and went with this instead.

In a script:

#!/usr/bin/env perl
BEGIN { do "./lib/reexec" for grep $_, $ENV{MATHPERL} }
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And in 'lib/reexec':

BEGIN {
    if ($ENV{REEXEC_LOOP}) {
        delete $ENV{REEXEC_LOOP};
    } elsif ($^O =~ /MSWin32/) {
        warn "Ignoring re-exec under Windows";
    } else {
        local $ENV{REEXEC_LOOP} = 1;
        exec($_, $0, @ARGV);
        die "Could not exec $_";
    } 
}
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I don't know how easy it would be to extend this to work safely under Windows: probably calling system() and then unmunging the exit value would work better then messing with ShellQuote.

Suggestions for improvement welcomed. :)